By Vidya Rajan, Columnist, The Times
I was working in my garden, doing the regular maintenance tasks of weeding and planning my fall plantings. I was planning on gathering seeds from some of my favorite flowers to replant. As I worked, I’d disturb a rock or something (a military joke) and find a germinating weed seed, or a cluster of ants eggs. They were everywhere. Seeds and eggs. Then it struck me that they are actually very similar. How so?
Seeds are what we call the eggs of plants: they are equivalent to the eggs that animals lay. They contain an embryo with a little store of food called the “endosperm” for it to use as it develops. Plant embryos have a root, the radical, that will grow down into the ground seeking water and nutrients, and a shoot, the plumule, that will grow towards light when the seed germinates. The little plant will then become self-sustaining. This is reminiscent of the development of the egg of an animal. The yolk and white provide lipids and proteins, respectively, for the embryo’s use as it is developing. After absorbing all the nutrients in the egg, the young then hatch out with different levels of independence depending on species. Chickens for example, hatch with their soft fluff, ready to start pecking their own food. (Just post-hatched chickens can be transported by mail in spring because they can survive up to 3 days without food and water.) Other birds are born naked, and have to be incubated and fed from the time they hatch to the time they fledge. Insects, fish, amphibians, and most reptiles emerge pretty much ready to look after themselves.
Plants do nurture their young, especially plants that spread by vegetative growth. Their offspring are usually attached at the root to the mother plant, drawing sustenance in the early days of their development. The purpose of seeds, though, is to conquer new environments, so most plants have seeds that are flung, transported, or blown by air currents to new habitations. But the earliest plants – the mosses and ferns – did not produce seeds, they produced spores. Seedless plants produce spores – these are embryos without food stores, and they need to set down in friendly territory immediately and germinate. There is some other curious stuff that goes on here – the spores can be either male or female, and they grow into little haploid plants (plants with only one set of chromosomes) which then give rise to little male plants and little female plants. The male plant produces sperm with tails, and they swim over to the egg contained in the little female plants. This swimming sperm explains why mosses and ferns only grow in damp and drippy places – those sperm need water for swimming.
The two plant groups that produce seeds are the naked seed Gymnosperms (ex. conifers and cycads), and the covered seed Angiosperms (ex. flowering plants). As an adaptation to dry land, gymnosperms and angiosperms got rid of the swimming sperm. The converted it to pollen by taking away the tail and encasing the erstwhile sperm in a sporopollenin coat which is waterproof. The ability to withstand desiccation makes pollen better adapted to dry land where water is not easily available. The pollen are lifted by air currents or by helpful pollinators to their tryst with the egg. Along the way, being cheap to make, plants churn out lots of pollen. Since the sporopollenin coat encloses cellular material made of protein, it is also nutritive. By fermenting the pollen grain into “bee bread”, bees can dissolve the cellular material inside the pollen with enzymes. The indigestible sporopollenin coat is excreted in the feces. That’s why bee poop is yellow – it’s from the hollow pollen shells being discarded.
The fertilization of a flowering plant’s egg by a pollen grain is a pretty amazing thing to watch (Although a little “teachy” this animation is excellent: https://youtu.be/bUjVHUf4d1I). Pollen grains have pores, which allows a tube to grow out of the pollen, carrying three nuclei. The first nucleus is the pathfinder and disintegrates, but the remaining two nuclei are the actual sperm nuclei. One of the two sperm nuclei fuses with the egg cell to make the zygote which will develop into the embryo, and later grow into a plant. The second sperm cell does what makes the seeds of flowering plants truly awesome: it fertilizes a pair of nuclei in the ovule called the “polar nuclei” which develop into the endosperm. In other words, this is the food packed in the seed for the embryo. Double fertilization which produced a three-nucleus fusion creating a “triploid” endosperm is an amazing and quite genius process I don’t have the space to go into – watch the video at the link above – but it ensures that only ovules that are fertilized develop into seed, a huge saving of energy for the plant. Unfertilized seeds are the duds you sometimes see when you are shelling peas. Gymnosperms, ferns and mosses don’t have this double fertilization process, so all their eggs, fertilized or not, become seeds. Their food is the matter the ovule is made of, haploid material, which makes for less nutritive food for the embryo compared to the triploid endosperm of flowering plants.
Add in a thick seed coat or some poison, and there you have the seeds of most flowering plants. Flowering plants don’t really want animals to eat their seeds. Many flowering plants produce edible fleshy fruits, with the seed being the hard bits that you spit out, or the poisonous bits you don’t eat. Take the familiar apple – the seeds of apple contain cyanide. The flesh is an alluring treat, but the seeds will kill you stone dead. At least, that was the case before plant breeders got into the act and selected for less toxin and more flesh. Even so, even now, 300 or so apple seeds will still kill you. Many seeds are very poisonous, the seeds of the castor bean being exquisitely so. They contain ricin, one of the most poisonous chemicals known, and a little amount – the size of a grain of salt – can kill. For a more detailed description of plant poisons, see an earlier Inner Nature article on Plant Poisons at https://www.unionvilletimes.com/?p=47676. Are there toxic eggs? Not by design, but a survey of free-range eggs from areas where organic pollutants persist in the environment showed that eggs can concentrate toxins (https://ipen.org/news/worlds-most-toxic-eggs-0). This is unsurprising: toxins in the environment in which chickens feed end up in the egg. If you are buying free-range eggs, check where the chickens live and what they eat.
Latency periods in plant seeds vary, but quiescence and dormancy are directly related to desiccation or the near-complete dehydration of the seed. Dormancy is retained until the plants rehydrate, although specialized extra demands such as vernalization and sunlight exposure may be required. These environmental cues work by breaking down an inhibitor of germination, and releasing the dormancy effect. Many primitive animals’ eggs also show dormancy [1]. Arthropods such as Daphnia, Artemesia, and copepods have eggs that undergo dormancy until environmental conditions are suitable. These dormant eggs are variously called ephippia, cysts and resting eggs, respectively which, despite being non-desiccated can survive hundreds of years. Although most familiar animals’ eggs do not have this ability to remain dormant, some animals have the ability to place their fertilized embryos in a state of suspended animation called embryonic diapause, usually when they are under stress. This is a reversible condition, and the embryo can resume development when conditions become favorable again [3]. This is the equivalent of dormancy in the seed because the embryo in seeds develops up to a point where a radical and a plumule are formed, and then goes dormant.
Those ants I disturbed took very good care of their non-dormant eggs. They needed to, but it was still sweet to see. Each ant picked up an egg almost its own size and scurried away to save it. It made me feel very tender and protective.
- Clark MS, D.N., Thorne MAS, Reinhart R, Drungoswki M, Albrecht MW, Klages S, Beck A, Kube M, Lubzens E., Long-Term Survival of Hydrated Resting Eggs from Brachionus plicatilis. PLoSOne, 2012. 7(1): p. e29365.
- Hartog, M., The Cambridge natural history. 1895.
- Fenelon, J.C., A. Banerjee, and B.D. Murphy, Embryonic diapause: development on hold. International Journal of Developmental Biology, 2014. 58(2-3-4): p. 163-174.
