Mid-April and the Ginkgos are flowering….well, technically not ‘flowering’, because they aren’t angiosperms. Botanically speaking, they are doing what they do instead, forming the little structures that contain their sex organs for what would most likely be failed attempts at reproduction. Think about it, in a community filled with males no progeny will be produced. We were on one of our walks down an inner section of Tri-Met’s Orange Line, approaching the Tilikum Bridge, when I noticed this event…I was a little surprised.
If you know much about Ginkgos you probably know about their fruit, which again is not technically a ‘fruit’. Only angiosperms, true ‘flowering plants, form ‘fruit’. The Ginkgo’s ‘fruit like’ structures are notoriously stinky when they become ripe, smelling like what many describe as being similar to dog ‘poo’, others liken it more to ‘vomit’, either equally unpleasant, when they fall to the ground and splatter or are stepped on…one of the reasons why these trees are cloned, grafted, by the nursery industry….By cloning selected forms propagators allow us to remove the chance of purchasing a female tree…unless in their zeal to bring a particular form to market they select a tree that hasn’t flowered yet….Without looking at their chromosomes, it is nearly impossible to determine the sex of a juvenile tree. Clones stay true to their sex, so if their scion wood, or buds, are taken from a male tree, the result will be a male clone. Ginkgos are a dioecious species, ‘di’ meaning two, so any one individual plant produces only male or female structures, so it takes two trees, of opposite sex, to produce viable seed. Monoecious means that an individual plant produces male and female structures. In Ginkgo spp. and the non-flowering gymnosperms these sexual structures are called stobili or singularly, a strobilus.
Understanding Ginkgo Sex…and Their Structural Variability
….To complicate this issue of sex in the plant world a little more, individual plants of a species possess either ‘perfect’ sexual structures, which combine both sexes into one, or ‘imperfect’ sexual structures which are either male or female….When imperfect flowers of only one sex are found on an individual plant, it is dioecious. When imperfect male and female structures can be found separately, on the same plant…it is monoecious…. A monoecious plant also results from having ‘perfect’ sexual structures. Dioecious, single sex, plants can only have imperfect sexual structures. Monecious plants, those with both sexes on each individual, can then have either perfect flowers or both sexes of imperfect flowers. Many or most Gymnosperms, including the Conifers, are monoecious. They tend to carry their imperfect male structures or strobili, higher up in the tree, with their imperfect female strobili below. The entire order of Ginkgoales were dioecious each individual having always been limited to either male or female, imperfect, strobili! Simple! Right?
We are therefore able to grow male trees for the landscape trade exclusively and leave seedling grown trees to natural stands or those unconcerned with the resultant smelly produce of female trees. A female tree, once it matures, produces ‘not a fruits’ whether there is a male in the vicinity or not! What! ‘not a fruits’ without sex! How can that be?…Don’t worry, the seed within these are generally inviable because having not been pollinated and fertilized, they lack the paired chromosomes of viable seeds. The fruit like seed coverings grow anyway.) We clone Ginkgos by grafting by grafting, taking scion wood from desirable male trees and grafting them to a Ginkgo rootstock. In this way we are able to select for both form of the tree and its sex.
Grafting a Ginkgo, gives a kind of ‘jump start’ to the new plant. For any one species there is a development process that occurs as it grows and matures, beginning at seed germination. For clones a certain amount of this maturation period is avoided by grafting. The progeny, the clones, will begin to flower in a shorter period of time than it would take had the tree been grown from seed. Because one of the criteria for an acceptable Ginkgo clone is its ‘maleness’ the original ‘mother’ plant must be a mature individual, otherwise the sex of the tree is not distinguishable. The scion wood, the material taken from the ‘mother’ and grafted on to the appropriate rootstock, begins from a more mature stage. This seems to hold true even when other ‘mother trees’, clones of the original, are the source of the scion wood, even if they themselves have not yet reached maturity. There will be a lag between grafting and when an individual tree begins to flower, but much of the maturation time can be avoided. This happens with other grafted plants as well and is why a grafted selection of a Magnolia species, or hybrid, for example, will flower much more quickly than a Magnolia grown from seed. Which brings me back to my noticing the flower like strobili on the Ginkgo clones planted out in ’14 along that section of the Orange Line…my brain wasn’t expecting to see such precocious sexual behavior from them.
Many of the large old Ginkgos in Portland are female. Those on the Plaza Blocks, Chapman and Lownsdale Squares, downtown, matured years ago and have been dropping their smelly not-a-fruits to the ground for years, most of it forming on wood high up in the trees where it is out of view. Their flower like structures are higher and further from view than on these ‘young’ street trees and their strobili are also quite small, tiny even, and individually inconspicuous. Working in Parks I really hadn’t noticed them before. These Orange Line trees are heavy with their catkin like male strobili this spring growing from the same buds at the terminal of stubby spurs spaced out along a branch from which the leaves emerge. Together with these leaves they look like little eruptions of green along a branch. This growth is somewhat reminiscent of genus Pinus, amongst the conifers, which grow their needles/leaves bundled in fascicles. Ginkgo trees have several unique or anomalous features when compared to their more contemporary and distant relatives, the massive group of flowering plants, the Angiosperms.
[Ginkgo biloba is a variable tree. If you walk through an old planting of seedling trees such as at Portland’s downtown Plaza Blocks, you will see this readily in their structure ranging from a very few branched gawky, large trunked forms to more broad spreading and dense structures with massive lower branches that nearly outsize the leader, to those with little more than a pole like single leader. Variability within the details of many species are often wider in more ancient lineages. It is as if the forms of early species were determined with a more loosely defined, broader set of genetic ‘instructions’ or, if you subscribe to the concept of morphogenetic patterns, a world in which those patterns are less defined, giving individuals of a given species a little more latitude, than those plants whose genetic paths were set many millions of years later. Whether branching is further limited by hormones released by the central leader’s terminal or the fact that some trees simply have fewer vegetative buds capable of branching, I don’t know. Regardless of why, planting a seedling Ginkgo is still a crap shoot.
There is something engaging, almost mesmerizing, about the shape of the fan-like leaves on this species that is singular, characteristic to this species alone, that draws people to these trees. Their gorgeous fall color, their habit of dropping their leaves cleanly and almost in a one note crescendo! Even the ease with which they can be raked up, draws admirers to these trees. I doubt that this species’ rarity in wild stands attributes much to people’s attraction to them…people simply like these. Because of this they are in relatively high demand and the nursery industry has been paying attention by being particularly alert to the appearance of seedlings that show characteristics that lend them for use as street trees, and there are several available to fit in a range of urban spaces, all tending to have a fuller, more balanced habit in terms of branching structure so buyers can be assured of a more consistent and balanced growing specimen. On the other end of the spectrum, of small, even dwarf trees, there are a great many cultivars, though with more limited availability, for the collector who has a smaller property. These include those whose leaves vary in form and color. For a sampling of street trees check out the catalog of J. Frank Schmidt while you collectors will have to look more widely. Here’s a list that gives a suggestion of the range of variability in the cultivars out there.]
Ginkgo: Its Decline from Dominance
What do I mean by anomalous? First, you need to understand that the Ginkgo is a relict, a survivor from a period many millions of years older than even the first Angiosperm species to appear a 100+ million years ago. The first Ginkgo species are thought to have appeared in the Early Jurassic over 200 million years ago (Mya). Earlier species have been found dated back as far as 300 Mya from which Ginkgo spp. are thought to have directly descended. At that time the two ancient plant orders Gingkoales and Cycadales shared a common ancestor. Bare in mind that our single remaining species of the this genus arrived much later in the process and that there were multiple other now extinct species in the group, a group that was wide spread around a very different world something we now understand as a result of our study of the fossil record and the history of plate tectonics, the movement of continents. They were once resident here in North America. (Check out the Ginkgo Petrified State Forest on Wanapum Reservoir and the Columbia River, out of Vantage, Washington.) Our single remaining species was eventually reduced in the wild to a small range in South-eastern China, though it was commonly grown in human settlements and around temples for many centuries across much of eastern Asia. There it survived in northwestern Zhejiang, a province which borders on the East China Sea, and south eastern Anhui, which shares a part of Zhejiang’s NW border, 2 million years ago…all other species being extinct by then. These few places provided ‘refugia’ where they were protected from the multiple cold cycles of the Pleistocene’s Ice Ages, and surviving in great enough numbers to maintain a viable population that could reproduce itself before ‘Man’ discovered it and began using it in landscapes.
Over this period it is generally agreed that there were some 20 separate cycles of glacial ice advances from the poles and later retreats, a time when those species, unable to ‘move’ to more hospitable climes, were either greatly reduced in numbers and range, or were extirpated, having gone locally extinct. The rugged geography of China created many pockets which served both to ‘trap’ populations or shield them in other cases. During the periods when the ice advanced, at its ‘peak’, conditions across the globe were colder and drier with precipitation severely reduced, much of the Earth’s water frozen, the oceans dropping as much as 100 meters, and average temperatures dropping 9º-18ºF worldwide. Summers were shorter and cooler allowing the accumulation of ice to build upon itself from year to year. This single species survived while retaining several characteristics that link it more closely to other ancient and extinct plants and help differentiate it from the more modern plants that dominate the world today. Its most closely related fellow survivors are the Cycads another once globally widespread genus, now increasingly in a precarious survival position within their much reduced historic ranges thanks in part also to the additional pressures of human development and, more recently, of poaching. In the past Ginkgo biloba was often closely linked to the seed bearing, non-flowering Gymnosperms, which include todays conifers among other families and plant orders, but many now classify it as the single remaining species of its genus, family and order. These more ancient plants were once well suited to a world whose conditions were generally both wetter and warmer than they are today. While Ginkgo trees are widely planted around the world they have not naturalized anywhere beyond their limited SE China range…their survival is dependent upon us. Fortunately they are a much loved and ‘iconic’ tree today.
Many plant taxonomists and geneticists have come to the conclusion that Gymnosperms and their most dominant representative, the Conifers, are also in decline, as the dynamic epoch long changes of the Earth shift in favor of the more modern Angiosperms, a group that still appears to be actively speciating and spreading. There has been a worldwide trend over many millions of years more ‘recently’ to more arid climates. (Yes, many conifer species, especially several of those in the Cupressaceae are well adapted to arid climates and grow best in them. I am writing of the overall group. I am also not ‘predicting’ the extinction of conifers just speaking to their decline from their once dominant position.) Gymnosperms aren’t going to disappear anytime soon, at least not on a human scale we can appreciate, unless of course our own activities bring this about. Plants respond directly to the growing conditions that they experience, but they also have a built in resilience attributable to both their relatively wide tolerance range to growing conditions and their ability to bridge across such periods, if they aren’t too long, by their ability to produce seed that can lie dormant, in many cases for very long periods, and to, in some species, their relatively long lives over which they can continue producing seed, even when conditions aren’t supportive of their germination.
In this plants can be more durable and successful survivors than the more ‘fragile’ animal species whose lives depend upon a continuous and regular supply of food, water and oxygen within a relatively narrow temperature range which, in we human’s case, we can moderate by wearing clothing, building ourselves shelter and utilize technologies to heat and cool our living space. These abilities are dependent upon our ‘economy’ and our ready access to energy and resources outside of ourselves. Plants must adapt to their conditions or die. We, to a certain extent, are able to ‘adapt’ our conditions to what suits us, while often in this process, changing the surrounding environment in ways unsupportive of the life that once thrived there. To underscore this many large mammal species went extinct over this same period of Ice Ages, unable to survive the changes themselves or the changed conditions around them that could no longer provide the foods they depended upon for survival, while a smaller proportion of plants are thought to have been lost. The smaller, the more numerous mammal species, had a greater chance of survival, their numbers insuring that enough of them would find safe routes to more acceptable southern areas during the advances of ice. As with any species enough individuals must survive to assure its continuation. When populations drop too low, a species genetic diversity, and thereby its viability, can be too limited and compromised, and the species dies out. Ginkgo biloba found a large and supportive enough refuge in China to sustain itself, that it did not find elsewhere around the world. Over its long existence genus Ginkgo has been successful here over 100 times longer than we much later ‘arriving’ human kind.
Ginkgo: Reproduction, Pollination and Fertilization
That Ginkgo spp. don’t produce flowers is not unusual. For the vast majority of time that plants in one form or another have occupied Earth, plants didn’t have flowers. Today this still includes the broad groups of Algae, Bryophytes, Lichen, Lycophytes, Ferns and Gymnosperms. These include many thousands of genera. Many others, including families and orders, have been extinct for millions of years having served their purpose while here. Each successive group has built on the past. Flowers provide a more specialized, complex and ‘contained’ structure in which reproduction takes place, a structure that does this by combining several ‘organs’, held separate in more ancient ‘lineages’. While pollination is still external to the process the other functions are now included within the flower.
Most significant of the changes manifested within the flower is the structure of the ovary, which is composed of one or more carpels that surround and protect the process of the growth of the female ‘gametophyte’, its fertilization and the development of its ‘zygote’ and endosperm. Within the ovary the seed develops. The ‘fruit’ that forms around the seed from the ovary also aids in the dispersal of the seed of many species, often being attractive to those animals that consume and spread it to other sites or forming ancillary structures that enhance their spread by wind or through the attachment, for a time to animals who will carry it far and wide. By containing more of the ‘processes’ and protecting them from the vagaries of a changing environment, these ‘flowering’ plants have changed both the structures and strategies for reproduction assuring them a higher rate of success in the world today. Flowers evolved as the Earth continued on its long term pattern of drying, enabling them to occupy more environments where pollination, fertilization and germination need to be tolerant of such dry periods. For much of Earth’s history the planet was wetter and milder…or barren of plant life.
Flowers come in a great many forms and sizes. Though we tend to think of flowers as those reproductive structures with showy petals, flowers include those structures of a great many species that don’t have petals at all or are much reduced. Many families, like the Poaceae, the grasses, don’t have flowers with petals or sepals while having other anatomical structures no other plant family possesses, but they are still ‘flowers’. They possess an ovary around which the rest of the structure is organized. Ginkgo spp. didn’t have petals either, nor are their ovules contained within a carpel or ovary. Their ovules are ‘naked’…exposed. Their male and female structures are much more ‘simple’ and ‘primitive’. The female structures of conifers and gymnosperms are aggregated into ‘cone’ like structures. The male structures are always less substantial than the female, generally falling away once they release their pollen. Both of these share a common physical structure of what is called a strobilus. These are simple aggregates of ‘sporangia’, along a stem. These strobili are present in Ginkgo spp.
The use of the terms pollen and ovum in explaining the sexual reproduction of plants is a simplification. Surprise! Surprise! the process is actually much more complex, less ‘magical’, but no less wondrous! Pollination and fertilization is not a one step process…plants are first pollinated and then, later. fertilized. The pollen ‘delivered’ to the female part of a flower where it later fertilizes the ovum or egg. The lag between the two can be considerable. Plants have produced multiple pathways to accomplish reproduction. Beginning from the mature plant, which botanists refer to as the ‘sporophyte’, because we must begin from somewhere, the plant prepares the way by producing ‘spore’ the tiny bodies that produce the even smaller gametes, either male or female. To do this within the spore it grows a ‘gametophyte’, in a sense a separate organism that will in turn form the sexual gametes through the process of meiosis and mitosis.
[In ‘earlier’, more primitive plants, before ‘seed plants’, this gametophyte stage was wholly separate and outside of the mature, sporophyte, ‘parent’. In these earlier, more primitive plants, gametophyte grow into a free living organism that has a very different structure than that of the mature sporophyte. I won’t discuss the particulars of how this happens here, but do understand that all spore and seed producing plants go through these two different stages, a gametophyte and a sporophyte stage, together referred to as alternate generations.]
Gametes are the sexual haploid cells that contain only a single set of chromosomes. They are formed by the gametophyte, developing within their tiny spores, in male plants called pollen, in female plants called ovules. These are the individual sperms and eggs that when united, through fertilization, will contain two sets of chromosomes, becoming diploid, and be capable of growing into a mature organism, a sporophyte, and be prepared to begin the process again, producing the spore inside of which will grow the tiny gametophytes. Sporophyte means spore producing. Gametophyte means gamete producing. Seed producing plants, which both Gymnosperms and Angiosperms are, include the gametophyte stage within their developing ovule and pollen. Seed producing plants, especially in a drier world, have an advantage over earlier plants in their ability to succeed. Angiosperms, extend this protection, by adding the protective ovary around its ovule and accelerating fertilization. Ginkgo spp. and the Gymnosperms leave their ovules ‘naked’ and exposed to the elements…and add a time intensive stage of having to produce a larger gametophyte ‘before’ fertilization.
Every species can have its own peculiarities in this process while sharing most of it. In Ginkgo their single naked ovule ripens into a drupe-like seed with a fleshy outer layer, its ‘not a fruit’, and a thin, smooth, cream-colored, hard inner layer. These drop free to the ground in the fall, long after their early spring pollination, after the first fall frost. At this time these seeds generally contain immature embryos and cannot be germinated. Embryo development continues while seeds on the ground are exposed to temperatures normally encountered during fall and early winter. Embryo maturation is usually complete about 6 to 8 weeks after the seeds drop (Lee 1956; Maugini 1965)
At present, the only stage in an Angiosperm’s reproduction that occurs ‘outside’ of the flower, is pollination. Pollination still requires the movement of pollen from the male to the female sexual structures, generally through the air though, in aquatic plants, it is often via the water. Plants that utilize pollinator species are more direct in this than their wind pollinated cousins, but distance must still be bridged. Because, unlike many/most animal species which are able to move, plants can’t position themselves to exchange gametes directly.
[Scientists have discovered, however, that certain Angiosperms are capable of a sexual shortcut…apomixis. These plants are capable of producing viable seed without pollination and fertilization. Their strategy is to bypass meiosis and form their embryo ‘spontaneously’. The resulting seed grows into genetic clones of the ‘mother’ plant, with the normal, diploid, or doubled chromosomes. The article I read stated that some 400 different species, 75% of which are in the Asteraceae, Rosacea and Poaceae, had been confirmed by 2004 as being ‘apomictic’, capable of producing seed without fertilization. It is thought that many more are capable of it but confirmation will take further research. The more you study living organisms, the more variations you find, the more exceptions to the ‘rule’. Will apomixis become the future…I would doubt this, because it halts the genetic mixing that would seem necessary to maintain a dynamic and diverse genetic pool. Maybe in some future epoch plants will become capable of sharing genetic material directly through their vascular systems grafted at the root level or there will be some merger of plant and animal species, not unlike lichens and plants will become capable of a sexual, movement, stage…or, animals, develop photosynthetic capabilities and produce our own compounds for growth rather than needing to consume them as foods acquired from other organisms…..Enough of this!]
The male strobili of Ginkgo spp.,form a chain like, catkin structure, of linked sporangia, and are capable of producing large amounts of pollen through active cell division or mitosis and, they in turn, male gametes. Life is busy inside the male sporangia producing the pollen within which the male gametophyte grows, which in turn produces the haploid male gametes, through the process of meiosis, which will ultimately fertilize their female counterparts, the egg cell, within the ovule or much larger ‘megaspore’. Botanists often refer to pollen as microspores and female ovules as megaspores. Producing a large amount of pollen is a necessary strategy in wind pollinated species of all kinds because much of their pollen will miss its ‘target’.
In Ginkgo spp. female strobili are small and inconspicuous, though still quite large when compared to a pollen grain, a quality, as they are not visited by pollinating insects, birds or bats is understandable, as a lavish and large display of a petalled flower would demand an expenditure of resources unnecessary and wasteful for the Ginkgo.
The female strobili contain a simple pair of ovules, exposed to the world having formed on leaf like structures. As these begin to mature one will be aborted while the other will grow and develop whether pollinated or not…into what botanists term a ‘megaspore’, still unfertilized. At the time of pollination the ovules are still quite small so an individual pollen grain when it lands on the ovule, will likely be near the end of the ovule where the pollen chamber will later develop. To increase the effectiveness of pollination the ovule secretes a mucilaginous substance which increases the chance that pollen will adhere to it and that it does so where it will be most effective, thus enabling later fertilization.
At some point the adhered pollen will more positively attach itself to the growing megaspore by growing a ‘tube’ into the forming pollen chamber along with a network of haustorium, tiny vessels, that can draw from the growing female megaspore, to sustain the male microspore. The pollen or microspore then ‘waits’. This is necessary as it will take at least 2-3 months, after pollination, for the female megaspore to grow and ready itself for fertilization. In some cases fertilization doesn’t happen until the megaspore, now much larger, has ‘ripened’ and fallen to the ground in all of its smelly glory. This delay is common amongst Gymnosperms though the time varies considerably. In many Conifers the ‘wait’ can take up to a year. By any measure fertilization follows pollination in Angiosperms much more rapidly as the ovary that will nourish the ovule is formed before pollination takes place.
Ginkgo spp. have always been wind pollinated not relying on insects or birds for pollination. This makes sense given their early evolution at a time when insect pollination did not occur. Remember there were no flowers nor a historical relationship between pollinators and plants when Ginkgo spp. first evolved…other than that many animal species consumed available plant tissues, exudates and metabolites. Pollinators evolved together with the arrival of flowering plants, the angiosperms, each adjusting to the other, evolving into what today can be highly specialized relationships, sometimes to the extreme of limiting a single pollinator species to the flowers of a single plant species! Prior to the early development of Angiosperms, plants depended on wind and water to move their pollen, to get it to the female gametes, the ovum or eggs, for fertilization. Think of the environments that algae (depending on the species algae either reproduce asexually or sexually by spores), moss and ferns often thrive in.
Fertilization happens when the pollen ‘finally’, releases its male gametes which go through an expanded ‘tube’ connecting them, grown from specialized cells that grew within the pollen. The male gametes ‘swim’ through the tube and ‘find’ their way to the egg cell to fertilize it held nearby in the archegonium of the female megaspore, where the developing embryo will grow. The male gametes ‘swim’ utilizing cilia or multiple flagella, much like the sperm does in animal species and lower more ancient plants. Recall that ancient plants lived in a wetter world and the male gametes originally developed having to swim, exposed, to their female counterparts. Cilia, and this strategy, are not used by modern flowering plants which quickly grow ‘pollen tubes’ down through a flower’s style, into the individual carpel and ovum through which the male gametes then move, two of them, in a process of double fertilization, one fertilizing the germ plasm in the ovum and the other initiating the growth of the endosperm, the ‘food’ supply for the later developing embryo that it will utilize at germination before it can begin gathering and producing the nutrients it requires through its own metabolic processes. There is little lag in angiosperms between pollination and fertilization. In angiosperms the female gametophyte is relatively tiny and doesn’t require a significant growth period before it produces an egg or female gamete.
The Ginkgo female megaspore is a scaled up version of its original self, the ovule, after one of the paired ovules in the strobile abort. The pollinated, but still unfertilized ovule develops on its own preparing itself for fertilization, growing its female gametophyte, which will ultimately produce a fertile, haploid female gamete or egg. Other specialized and unfertilized cells inside the ovule will grow into the endosperm which will nourish the germinating plant very much as does the endosperm in Angiosperms. (A note on the ‘nut’ within the fleshy covering of the ripening seed that are sometimes consumed as a delicacy in east Asia.)
Botanists believe that both of these reproductive structures, male and female, have evolved from leaf tissue, a further elaboration of the reproductive structures found on the fertile fronds of ferns (like that alliteration?). They genetically traced this development through the long extinct genera they’ve identified in the fossil record to an intermediate group, the ‘seed ferns’, now long extinct, that at one time were abundant.
Ginkgo spp. have always been seed producing plants so they’ve had a greater ability, in general, to adapt to dry periods than plants that rely on an external gametophyte stage subject to the more widely variable conditions of the environment. There are ferns which have adapted to very arid climates today, and yes, yeasts and fungi as well, but ferns, as a group, first evolved in much moister environments which supported their external, moisture dependent gametophyte stage.
Early, non-seed producing plants, all went through variations of the external gametophyte stage, In some genera, the non-vascular, bryophytes, which include mosses, liverworts and hornworts, and those species of algae which can reproduce sexually, it is the gametophyte stage which is dominant, the form with which we are more familiar, because these organisms, these plants, spend the majority of their lives in this stage. The ‘higher’ vascular plants, including the oldest surviving group of them, the lycophytes, which include the club mosses, quillworts and spike mosses plus a number of extinct groups such as the scale trees, together with the ferns, gymnosperms and angiosperms, have a dominant sporophyte stage. The line that life follows is not as ‘straight’ as most would like to make it. While life does follow particular patterns there has always been a high degree of variability within those patterns…without this variability there would be no evolution and life as we know it today would have never occurred.
Modern, seed producing plants, have internalized the gametophyte stage, removing more of the processes chanciness, assuring that more of a plant’s energies are productively spent in successful reproduction. There remains, however, a great deal of chanciness, redundancies, which help nature continue despite suffering catastrophes of great and smaller scales. A single ovum and its fertilizing sperm are a very long way from reaching the stage of a mature reproducing individual, a reality this life also relies upon, because if every individual gamete, every fertilized egg cell, were to mature successfully, the world would have overpopulated and exhausted itself many millions of years ago. We must also consider that much of this ‘lost’ life serves a very necessary purpose by its sacrifice as a food source for other life and a role in maintaining and building the soil upon which all of life depends, capturing and holding the Earth’s storehouse of nutrients.
Life requires failure. It requires death. Sexually reproduced plants go through the two stage process of alternating generations…gametophyte to sporophyte and back again. (Single celled organisms rely on simple cell division, without a sexual, mixing of genes component, and so don’t go through alternating generations. They have other proven strategies which have enabled them to adapt, several which have been instrumental in the evolution of ‘higher’, more complex organisms which lead to the Eukaryotic cell, the basic building block upon which all of modern life is dependent. Alternate generations is a cyclic pattern of an asexual stage, followed by a sexual stage. Meiosis and cell division followed by fertilization and the development of an ‘adult’ in an endless cycle. It wasn’t my intent to confuse anyone here, but I probably did. I remember getting thoroughly confused in my high school biology class on this with meiosis, mitosis, gametophytes and sporophytes!!!!
What? there’s more? Ploidy refers to the number of complete sets of chromosomes within an organisms cells. In a species’ gametes, the sexual cells, have been reduced by half in a process we label meiosis. These join together, forming the diploid zygote with two complete sets of chromosomes, which precedes the growth of the embryo which then is capable of actively multiplying, differentiating as it grows into the adult sporophyte. These terms, the technical ‘jargon’, help us identify what is happening. This process is what we see as normal development…but development is not always normal. These days we ourselves often push life into the abnormal range…intentionally for our own purposes. Often times, when growth is abnormal, an organism dies, because its integrity is simply too far beyond the bounds of normal, out of balance, to survive, yet in other cases it does survive and sometimes quite well.
For whatever reason, and these can be varied, and their causes are not fully understood, embryos form with abnormal numbers of chromosome sets. Sometimes meiosis is interrupted and gametes form with more than one set of chromosomes and sometimes these survive beyond fertilization and grow into mature organisms. Individuals can have 3 or 4 sets of chromosomes…sometimes, more rarely, even more sets. There are theories out there that attributes at least part of reason for speciation, the formation of new species, and the process of evolution to this process of polyploidy or ‘whole genome duplication’. It is thought that these duplicate genes can give an individual developmental advantages, or disadvantages, that can change the lineage that follows…if it is able to reproduce.
There is another part to ploidy that has occurred in genus Ginkgo, though it is thought to be rare. It shouldn’t happen at all, yet individuals sometimes develop which are ‘haploid’, having only one set of chromosomes…like a normal gamete. Evidently, sometimes a female gamete, must develop without having been fertilized, producing a seed which grows into a mature sporophyte. This is extremely rare. The resulting offspring generally exhibit morphological differences from the norm. This is consistent with what plant breeders have long known and why they sometimes attempt to induce polyploidy to produce changes in the offspring. In genera, like Iris, this has been done to produce plants they may be larger in their entirety or have larger flowers, or flowers with more than the normal number of parts, such as petals. Such polyploids are often sterile as they have ‘problems’ later when ‘trying’ to reproduce, meiosis failing or producing gametes that are unsuited and incompatible with gametes of ‘normal’ plants. In the case of the above very rare haploid individuals, meiosis can’t occur at all because with only a single set of chromosomes, there is nothing to split….yet haploid individuals still sometimes occur. While confirmation work is incomplete, polyploid plants do exist and there are characteristics which are attributable to their chromosome ‘imbalance’, consistent with what breeders have found in other genera. If a diploid plant is normal, haploid plants have been found which tend to be dwarf and can have smaller than normal leaves. That being said, not all dwarfs are haploid, though my own, G. b. ‘Chase Manhattan’, is. Going the other way, increasing ploidy, has been shown to increase leaf size as well as its shape. Triploids, those with three sets of chromosomes tend to have abnormally large leaves, while tetraploids, those with four sets, will tend to be larger yet and even develop more splits or lobes in their leaves These plants, in nature, would tend to be ‘dead-ends’, sterile, live out their lives and then die without having reproduced. In the landscape, however, they continue, even increasing in number when we intentionally clone them. These clones, have been selected by us. Through horticulture we are changing the evolutionary possibilities. though the odds of my G. b. ‘Manhattan’ having much influence on the future is likely vanishingly small.
When you look at Ginkgo reproductive structures, remember they aren’t flowers, they are an intermediate step in a continuously evolving process over an unimaginably long period. The male structures are held in an aggregate that appears much like what you might see on an Alder tree, with its clusters of tiny male sporangia held in strobili, but again, they are ‘alike’ only in gross appearance. Each of the little bodies in these aggregates are a male sporangia which will produce and later release pollen, in which grows a gametophyte that produces the sexual male gametes. Like most wind pollinated plants much or most of the pollen will drift off target and come to naught. As most people plant male clones, there is little to no chance of producing seed and the seedlings that could potentially follow.
Plants like Ginkgo biloba are relicts, and I find them fascinating, in terms of their place in the evolution of plants, and the questions they raise for the plants we live with today, as well as what they say about the fate of those long gone. Evolution continues around us in a constant process of selection and presumable speciation. It makes me wonder what one day will come and I do so not in fear, but with a sense of anticipation. One day each of our lives will end. It has always been this way and must continue to be. We are not at some ultimate end point today. We are participants in something so much bigger than we can ever fully appreciate, but that is no reason to stop ourselves from attempting to do so and taking the time to marvel at what continues to take place around us. We should all strive to be more conscious and active participants in its unfolding. The more I look at plants the more I see them as wondrous, fantastical organisms. They often draw me deeper into the world, a world that so many of us seem oblivious to, taking it for granted as a static and too common given.
A Few Source Articles
For those of you eager for a closer look at all of the technical bits discussed above without going completely into the deep end…check this link out! Skim the denser, more technical sections. There is much of value in this. Reviews like this one, give us a better idea of the almost stupifying complexity of such a simple organism, causing us to wonder whether a plant’s genetics can possibly determine the placement of every cell, as well as the organism’s continuing growth and metabolic functions. It gives one pause and support to the idea that there are larger morphogenetic patterns that play a role in the form, function and growth of organisms.
Curious about the end-around that some plants are able to accomplish via apomixis, try this article.