Frost in May – still not safe to put your seedlings out! The unusual weather was taxing green-fingered members of one science discussion group. How do perennials survive, one asked – antifreeze? It’s true, the sugar sucrose or larger protein molecules act as a kind of antifreeze, lowering the temperature at which water freezes in plants, as salt does when sprinkled on ice.
Another wondered how annual plants ever get to succeed in the long run. If they fail to make it through the winter why haven’t they died out? No sooner said than answered: of course, they sow their seeds. And that’s where the sex comes in.
Reproduction
Perennials have the advantage that they can reproduce themselves ‘vegetatively’, by means of bulbs or rhizomes for example. Bulbs are made of modified leaves which get added each year forming the familiar rings in onions. A key function of the bulb is to store food, including the sucrose that prevents them freezing. But it also contains the miniature bud of a flower inside, just waiting to begin growing
A rhizome differs from a bulb in that it is essentially modified stem, rather than leaf. It runs horizontally underground. New shoots can grow upwards from it and roots, downwards. Like a bulb, it stores nutrients that see it through the winter when the upper parts of the plant die off. Ginger is a good example of a rhizome.
Picture credit: Sengai Podhuvan
A tuber is simply a section of a rhizome that has thickened to store extra nutrients, particularly starch – a large carbohydrate molecule that holds plentiful energy between the atoms of which it is composed. Potatoes are a good example of a tuber.
Apart from the all-important nutritional value that onions, ginger and potatoes offer, these forms of perennial growth share an important botanical property: they can all get by without sex. Cells similar to our stem cells are able to divide producing daughter cells which can go on to become any kind of specialised cell. These generic ‘meristem’ cells enable the various kinds of cell needed by the tissues of a plant to develop and grow: leaves, stems, flowers and so on. It is meristem tissue that enables stems to produce off-shoots and buds to produce flowers, for example. We humans are not able to grow extra limbs at will in the same way, though as embryos our stem cells were once crucial in sprouting our limbs and generating the various kinds of specialised cells needed for a fully formed baby.
What’s the point of sex?
So, if bulbs and rhizomes get along fine by just growing new cells from old – the newborn springing vegetatively from parental tissue – why don’t we all do it: humans, animals and annual plants? Why do we have to go through such complicated mating processes to bring on the next generation?
Some species reproduce with sex, some without and others are flexible, capable of either. As both methods have survived the rigours of evolution, there must be pros and cons to each. The arguments against sexual reproduction are pretty straightforward: finding a mate can be time consuming, but reproduction cannot occur without one; the process can consume precious energy and the complexities of fertilisation mean it doesn’t always prove successful. So clearly there must be some strong points in favour for sexual reproduction to have taken off in the big way it has.
The overwhelming advantage flows from the variation it introduces into the set of genes (the genome), and hence the whole body of the offspring. Diversity of genes, and of the physiology and anatomy they go on to determine in the individual, can be highly protective. Over-reliance on one specific variety of potato in Ireland in the 1840s led to famine when a particular microorganism was able to blight the entire crop. Any change in the immediate surroundings – food sources, predators, microclimate, disease carriers – that proved deleterious for one individual would immediately threaten the whole population. Variation, on the other hand, means that slightly different individuals are produced with each offspring and different individuals may have different propensities to survive. In the long run, survival is not just good for the individual and their group, but also fuels the evolutionary process by favouring the propagation of those characteristics best suited to their environment. A particular giraffe that, by random chance, had genes that confer a slightly longer neck would reach the leaves other giraffes couldn’t reach. The survival prospects of its genes would improve, as would those of its enlarged family of descendants.
Sexual reproduction
Plants and animals have differing ways of handling sexual reproduction, but some fundamental elements are common to both kingdoms. The key concept is that characteristics from each of the two parents get mixed randomly as they are passed on to the offspring. This mixing occurs, of course, in every generation: each parent’s sex cells – sperm and egg – were themselves created from a random mix of characteristics from their parents; and so on back through the ancestors.
But what exactly do we mean by the rather vague word ‘characteristics’? Our bodies – and the forms of other animals and plants – develop their shape and functioning thanks to the work of several types of molecule: notably proteins, hormones, fats and carbohydrates. Proteins are made within the cells of organisms from the material they eat using the recipe encoded in their genes.
A gene is a short stretch of the very long DNA molecules that lie in the heart of almost every cell of our bodies – inside the nucleus. Each gene contains the recipe for making a specific protein. Proteins, produced in their billions inside cells, go on to make the tissues of plants and animals and the enzymes that create and regulate them.
The act of reproduction is the means by which genes from one generation get transmitted to the next. The selection of genes that get transferred, determine the forms and functioning of the new generation. Thanks to sexual reproduction, in contrast to the asexual cloning or vegetative forms, each offspring has two parents. Each parent supplies just one version of each gene from the two they have and it’s more or less random which version gets transferred, introducing variation. Further variation follows as the genes, packaged up in the larger structure of chromosomes, get mixed together randomly when the sperm and egg cells fuse.
The remarkable consequence of these random mixing processes is that, although each offspring carries the full set of genes for their species – thereby ensuring that all humans (or worms, pigeons or geraniums) resemble others of their species – each individual offspring is nevertheless unique. With over 20,000 types of gene in the human genome, random mixing of different versions from each parent means the number of permutations and combinations is almost infinite. That’s why we all look slightly different.
How plants do it
School biology lessons in the 1950s were hardly riveting in my personal experience. There was one major theme we wanted to know about as young teenagers and pistils, pollen and stamens were not it. How misguided we were! Reproduction in plants is an amazing story in itself which now, I realise later in life, lies behind the fascination gardeners and nature lovers have with flowers, seeds and propagation. To begin with it’s so varied.
First there’s the option to dismiss sex altogether as rhizomes can choose to do. Thanks to the meristem cells, they can divide to produce the specialised daughter cells needed for making buds or leaves, roots or shoots – a process triggered by hormones and, in some cases, in response to temperature changes or the gradual variation in day-length. Plants like potatoes or strawberries or onions that can grow in this vegetative way can also go on to flower and reproduce sexually. Many other types of plant don’t have the vegetative option: sexual reproduction is their only route.
However, unlike so many animal species, the male and female parts are both found together in the same plant, in its flower. The option is therefore available for pollen from the male part (stamen) to transfer directly to the female part (pistil) so the plant fertilises itself.
Picture credit: Anjubaba
Self-pollination like this does indeed happen but many species have evolved to minimise the chances of it by physically separating male and female parts (as in cucumber) or arranging them to mature at different times or be of incompatible shape or size (primroses, for example). The reason for this we have already encountered –more limited variation when genes from both male and female parent come from the same individual plant. With less variation in the gene pool, the greater the risk of offspring susceptible to disease or degradation.
Cross-pollination ensures that reproduction involves different genes from different parents: pollen from one individual plant fertilises eggs from another. As a result a greater range of versions of each gene are mixed in producing offspring. The chances of feeble progeny are reduced. But how does the mixing take place at all, given that plants are largely immobile and don’t get out to meet their partners? Evolution has resulted in many ingenious ways of overcoming this mobility problem.
One involves a helpful force of nature – the wind (where it prevails). For many types of grass and tree, lightweight pollen on the outer surface of flowers gets caught by the wind, blown about and, with luck, falls on the female parts of an individual of the same species, elsewhere. Such wind-borne fertilisation requires no flowery means to attract a pollinator – bright colours or seductive scent: the wind either blows or it doesn’t. To maximise its chances of success pollen usually appears before the leaves which can impede the passing air currents.
Picture credit: Trish Steel
The most obvious vectors for moving sperm around are of course animals. They are not only mobile but, unlike the wind, are open to persuasion. A sweet smelling nectar and a brightly coloured flower are all it takes to attract bees, butterflies, bats and birds who through their peregrinations can connect up distant reproductive partners. The interplay of winged beasties with the sex cells of plants is an inspiring example of collaboration arrived at through evolution.
Flowering plants produce energy-rich nectar that fuels the pollinator, while hairy parts of the legs and underbellies of the creatures grasp the pollen grains conveying the vital genes from the male part of the flower.
Picture credit: Lars Falkdalen Lindahl
Co-evolution has not only brought this mutually advantageous system about, but has gone on to develop a close match between the mouthparts of the pollinator and depth of the flower. This not only allows for more efficient transfer of pollen but also prevents other freeloader species from raiding the nectary. This precise matching of flower shape with pollinator may be one of the factors driving the evolution of such variety in flowers.
Flowers that attract bees for example are likely to be blue or yellow as the bee cannot see the colour red. Those pollinated by moths tend to be flat, allowing the insects to land comfortably. Flowers that emerge at night, mainly in hotter countries may use bats for pollination; their flowers tend to be large and white to stand out in the gloom. Flowers such as orchids which rely on birds like the hummingbird, are shaped in such a way as to enable the bird to approach the flower without catching their wings in it.
Some flowers go so far as to attract a male insect by mimicking the appearance and smell of a female. In attempting to mate with this sham partner, the insect inadvertently picks up the vital pollen.
Photo of a bee orchid by Bernard Dupont
Offspring With the male genes of one flower artfully deposited on the female parts of a neighbouring one the process of fertilisation takes place. The sperm and egg cells fuse inside a part of the ovary called an ovule.
The latter develops into a seed, comprising an outer coat which protects the all-important embryo and the food store upon which it will initially rely. The ovary itself, which houses the ovules, develops to become the fruit, acting as a protection and a source of nutrition for the seed.
The job of the seed is to nourish the embryo initially, disperse itself to a suitable environment and lie dormant until conditions are favourable. Dispersal again reflects the marvellous possibilities of co-evolution. Some are light and have wings or hairs that catch the wind. Others are buoyant and float away in streams; many have attractive fleshy parts that get eaten and excreted some way away and some are sticky and attach to passing fur.
During the dormant period life inside the seed grinds to a halt. It awaits the arrival of water, oxygen, light and warmth. Water softens the outer coat, enabling the embryo inside to swell and split open the coat. Oxygen absorbed from tiny spaces within the surrounding soil enables respiration to begin, releasing energy for growth. Different varieties of seed respond to temperature and light in different ways. Some require warmth to germinate, others cool, some require a certain level of light, others do not. Temperature affects the rate of metabolism – it can be too fast or too slow. Absence of light can cause a seed to wait patiently in the dormant state, as light would be essential for the growth of the subsequent seedling by capturing the Sun’s energy through photosynthesis.
Conclusion
This story of reproduction and it’s intricacies tells us something of the way in which life is perpetuated: in the short term, by bringing on the next generation and, in the very long term, by evolving new species. The form and functioning of every individual, from a single-cell amoeba to you and I is determined (largely) by the information encoded in our genes. Duplicating this in a clone enables individuals to reproduce relatively quickly (like bacteria) but can spell disaster for a population if and when the environment changes.
Sexual reproduction has evolved through better survival rates when different versions of genes are available from each of two parents. But the persistence of both sexual and asexual or vegetative reproduction in plants, fungi, sponges and various simpler forms shows that the balance of advantage is a fine one. For Homo sapiens and most animal species, however, the only way is sex.
©Andrew Morris 19th May 2021
Getting to Grips with Science 