(Reading time: 15 mins.)
When plants first “crept” out of the sea and freshwater on to land, it was a perilous undertaking. The shore and in particular the sea shore is a very hostile environment, subject to battering and scouring action of wave, wind and wind-driven sand, alternating between inundation and desiccation and even both in the same day. Those early plants were not just explorers but colonisers and innovators; many died but those that survived changed the world, its very earth and atmosphere.
There are about 320,000 known species of plants, a total that does not include most hybrids, sub-species or selectively-bred varieties. Botanists exclude from the term “plants” some of the green and all of the brown sea algae as well as the fungi and bacteria. The vast majority of plants are coloured some variety of green because of the action of photosynthesis inside them, which attracts the blue and red ends of the light spectrum but does not absorb green, which is why we see them in that colour. Some 260,000 to 290,000 species produce seeds but algae does not. Mosses and ferns, which are plants, produce spores instead, in common with fungi (which however are not plants).
We study life to place it in an order, to simplify understanding but life diversifies into a huge array.
Plants are pioneers, colonisers, innovators and builders at least comparable to the animal kingdom, to which they are related and, I would argue, with a superior record.
Plants first “crept” out of the sea and freshwater during the Ordovician period, around 470 million years ago; they were non-vascular (without “veins”) and without roots, like mosses and liverworts. It was a perilous undertaking. The shore and in particular the sea shore is a very hostile environment, subject to battering and scouring action of wave, wind and wind-driven sand, alternating between inundation and desiccation and even both in the same day. Plants on land carry the genes of the early explorers, pioneers, survivors – high in endurance, adaptability and innovation.
But why abandon the seas, lakes and rivers in the first place? Presumably there is always a pressure in nature to explore niches and new territory, thereby escaping pressures of population, predation, competition and consumption of available nutrition … And while some life-forms specialise in particular environments and nature also pressures in that direction, ultimately that is a highly dangerous strategy, general adaptability to food sources and environments being the best bets for long-term survival and multiplying – as shown by homo sapiens, for example.
First ashore, establishing a literal (and littoral :-)) beachhead, might have been a kind of algal slime. Perhaps it survived only while wet, died, was replaced by other migrants …. but probably at some point some carried survival pockets within them, able to regenerate when moistened anew. Or it might have been some moss or liverwort, later a branched and trailing plant but dealing with the same problems and developing a similar strategy for survival.
We can imagine a conversation, in which one plant organism on the shore questions another:
“It gets so dry here I feel I am going to wither and blow away.”
“Just hang on there. We’ll get rain soon. And there’s always dew at night.”
“I can hardly wait. Remind me why we didn’t stay where were were, with all that lovely moisture.”
“Getting eaten by other life-forms. Competition for light.”
“Oh, yeah. Sometimes I forget.”
REACHING DOWN, STANDING UP
In lakes, plants could simply float upright in the water reaching towards the light (and avoiding being covered in sand or silt) as many water plants do today, or on the surface, as algal mats and bloom do, or for example the various types of “duckweed” that not only float but multiply to cover the whole pond surface. In the sea and in fast-flowing rivers however, fixed plants needed to grasp surfaces and developed means of doing so; but these were not roots as such – more like anchors. Later, as they colonised the land, most plants did indeed develop roots not only to anchor themselves in the ground or to cling to difficult surfaces but also to bring up water, the tap roots for this purpose often going quite deep. Roots also brought up nutrients.
The roots also made it possible to cling to inhospitable surfaces, including even the perpendicular or overhanging and also to exploit cracks and fissures by tunneling into them. In the course of this activity, plants changed their immediate physical environment, by helping to break down stone and also by trapping material blowing in the wind.
But why set up home clinging to a cliff or today, a wall or a chimney stack? Well, plenty of sunshine, for one thing, no competition for another! Of course, not much soil there or even none at all for nutrition – but still, most things in life are a trade-off, right? How did the seeds get up there in the first place? Wind … or birdshit.
Of course, some of the colonisers developed other ways to cling to surfaces, as was the case with the mosses, lichens and liverworts. And they also trapped material and contributed their own to it as they died, regenerated, died …. But without roots that only works when you keep low and hug the ground. If you want to grow tall to reach for sunlight and if you want to exploit soil, you need roots.
Plants at first fed almost exclusively on sunlight it seems, broken down into sugars by chlorophyll in photosynthesis. But those that developed roots also, probably as anchors to prevent themselves being blown or washed away, or to help them grow tall and compete with other plants to catch the sun, learned to draw up water and to feed on nutrients in the soil – phosphates, nitrogen, potassium etc. Some, like the legumes, beans, peas and gorse for example, even learned to extract one of the gases that make up air, nitrogen and, with the help of a bacteria, to fix and store nodes of it around their roots.
Once you have roots, why not grow stems, branches, trunks, whereby you can reach higher and higher, for more unimpeded sunlight and outpacing the competition perhaps. Your building material will need to be tougher, especially for trees, bushes and shrubs, to bear the weight, withstand the winds …. but flexible enough to stretch as you grow. Having the ideal material already in cellulose, all that is necessary is some kind of hardening process. A plant might explain to puzzled humans: “Think of keratin and how the same basic substance has been used to make stuff as varied as feathers, fur, human hair and beetle carapaces.”
If you were a plant that had learned to spread fast over distances to catch the sun, covering ground and clambering over obstacles, you might find one day that there is another way to reach towards the sun – climb up the plants that are reaching up there! Don’t invest in slow build-up and hardening of cellulose – go for fast growth and gripping or winding ability instead, or turn some of your leaves into grasping tendrils. Some climbers such as lianas in the tropics and ivy and honeysuckle in Ireland, are perpetual climbers, remaining in position throughout the year (although the honeysuckle will lose most of its leaves in the Autumn) and extending during the growing seasons. Others climb only in the Spring and Summer and die afterwards, for example bindweed and runner-beans.
Most plants have leaves, which is where the photosynthesis takes place; they are in fact sunlight collectors and the plants deploy them to best effect to catch the available sun. Quite a late development, they were flanges on the stems first before becoming appendages further out of the plant’s main body. Most leaves are intricately veined and contain many different layers and parts and it is within them that photosynthesis takes place but strangely, they are mostly short-lived and in cold seasons even in perennial plants, with a few exception, all but the conifers let them fall.
The greater the volume of material created by plants, the more there was to decompose with their deaths or seasonal decline. Bacteria, already long existent on the planet, evolved to feed on this detritus and break it down into soil, which the same plants or others could turn to their advantage as a medium in which to anchor but also from which to draw nutrients. Other organisms evolved to live on and break down cellulose too, the main building material of plants: fungi, gastropods like snails and slugs, woodlice, termites …..
The plants, with the help of bacteria and other organisms, were creating the environment below them!
But they were and are doing more than that: they are also creating an environment immediately around them. The most concentrated examples are perhaps rain forests, tropical, temperate and cold-climate, retaining a surrounding moisture-laden air, in which not only the local tree species thrive but also providing ideal environments for ferns, algae, orchids and epiphytes and, of course, mosses.
Away from forests, sphagnum moss creates a mini-atmosphere around itself and as generations die, their bodies create a spongy moisture-laden medium. This bog is quite capable of existing on an incline, with much of the water being retained by the vegetation and ‘soil’, as may be seen in a number of examples in Ireland, such as parts of the Dublin and Wicklow Mountains.
Plants, especially trees, discharge oxygen into the air and consume carbon dioxide during the daytime, for which reason they are sometimes called “the lungs of the world”. They have not only created an environment for themselves, below, around and above but also for so many other life-forms – including ourselves.
LEARNING TO LIVE IN DIFFERENT CLIMES
Plants that adapted to grow in arid areas developed fleshy ‘leaves’ and often stalks, in which to store water and sometimes long tap roots to find that water. But extensive shallow root networks are good too, to collect the occasional rain water that is quickly absorbed into the soil or otherwise evaporates. The “pores” on leaves through which plants absorb carbon dioxide and allow the gas-exchange necessary for photosynthesis (stomates) also permit evaporation of water, hence many dry-condition plants have fewer of them. Some only open to collect carbon dioxide in the cool of the night and store it for use on the following day. Plants grow trichomes, tiny bristles, underneath their leaves but some arid-dwellers grow them also on top of their leaves; these ‘trap’ a layer of air that prevents or slows evaporation.
In very wet areas, plants learned to remain active by a number of strategies. Of course they originally came from aquatic environments but for some of them, returning there again after adapting to dry land, produced challenges (think of the changes necessary for land mammals to evolve into seals, otters, dolphins and whales). Nevertheless we have lilies growing in shallow water with wide floating leaves, rushes with upright blade-like leaves growing inside the water margins, thin spears of rushes in damp and water-logged land. That too is the preferred environment of some other plants and grasses, including the rice plant. And of the willows, alders and hazels growing on the banks and stabilising them. In the tropics and semi-tropics, mangroves do a similar job to willows but on a much grander scale – and they tolerate seawater too.
The alder, a tree with a high toleration of water around its roots, is thought to have been the major post-glacial coloniser of Ireland, following the retreating ice across the land. It is the only native tree which though not an evergreen produces cones, an indication of its early adaptation to cold climate. Cones, when closed, protect the seeds inside against continual freezing and thawing and, when the cones begin to dry and automatically open in spring and summer, allow the seeds inside to drop out to the ground, to be carried by river or on the wind. A closed cone collected and brought home will open as it dries; shake it then and the seeds will fall out. Alder timber, incidentally, remains waterproof for centuries, witness the wooden piles in Venice.
Adapting to cold seasons required protective materials, structures and timing. The deciduous trees (and it is worth noting that many trees have both a deciduous and an evergreen version for different climes) shed their leaves and close down for the winter, the sap retreating down to the roots. Were the sap to remain in the exposed branches it would freeze, expand and destroy them. The leaves drop because they no longer receive anything from the tree; it is going into a kind of hibernation, in preparation for the coming winter.
Many of the conifers have downward-sloping branches, to allow most of the snow to slide off, rather than break the branches with its weight. People who live in areas with heavy snowfall also tend to live under sharply sloping roofs. The “leaves” of the conifers are small, narrow and hard so that most snow falls through them and are also covered in a waxy polymer to withstand freezing. The plant cells can be emptied of water to prevent freezing but a dense waxy residue keeps them open for refilling. So, of course, they have to be tolerant of dehydration. Concentration of sugars also lowers the freezing point and small flexible conduits for water resist the formation of large ice bubbles that can burst those “pipes”.
Seeds know which way is “up” and which is “down”, which is quite an amazing thing; the tap root of a seed, germinating in the dark, goes downwards while its shoot grows upward.
In fact, the plant seed also knows the right time to germinate – too early in many climes and it will be killed by frost, too late and it will have insufficient time to develop before the next cold period or will be unable to compete with other seeds that sprouted earlier, depriving the late-comer of sunlight and possibly nutrients. The decision is made by a number of factors feeding into a small cluster in the seed tip, consisting of preventative and initiator command centres. When the initiator section’s hormones exceed that of the preventative, it is time to germinate. Not very different from our brains’ decision-making process, is it?
Also, cut a living branch and often the plant will mobilise to produce one or more shoots at the cut-site. But should that cut be enclosed within soil, the tree or bush will produce roots instead – it ‘knows’ the difference. This knowledge the gardener takes advantage of when she “air-layers” a shrub or tree by nicking a branch, then covers the cut with soil wrapped in a plastic bag, waiting for a root to develop and then cutting the cloned sapling free, ready to plant.
Without eyes, plants are also capable of detecting where the light is; if one places a climbing plant seedling in a dark cellar with a small window high above, the plant will climb towards the window, striving to reach the light. The sunflower and the flowers of some other plants turn towards the sun, following its progress across the sky. Many flowers, including those of the dandelions and daisies all around us at this time of year, close when the day ends. A “Swiss Cheese Plant” I once had managed to slip one of its suckers — like a long surface root — down the back out of sight and when I eventually discovered it, the sucker had gone under the carpet and had extended around six feet towards the window.
Of course, it may have been searching for moisture.
Plants can sense moisture and do go looking for it, something at which eucalypts are particularly adept. Unfortunately, this can cause problems for other trees and shrubs growing in the same area, as the eucalypts suck up the water from greater depths (the eucalypt doesn’t care however nor do some of its planters). During the severe drought in parts of the USA last year, it was reported that trees were breaking open water pipes with their roots to get at the precious liquid. It appears that the reports were mistaken but instead the roots were extending towards the detected moisture from leaks in the pipework. Of course, then the roots might widen the gap ….
Some plants at least are also ‘aware’ of being attacked, for example by an infestation of caterpillars. Those that have reserves of a defensive poison at their disposal are not only able to deploy it but also to communicate to other nearby trees of the same species, so that they too deploy the poison – before the caterpillars have even reached them! It is thought that the trees communicate underground, through their roots.
Plants also know when their offspring have reached enough numbers and a sufficiently advanced stage so as to put their energy into maturing them, rather than producing more growth or even more seeds. Presumably they receive a chemical signal when enough roses have bloomed, been fertilised and the rose hips, the fruit containing the seeds, are swelling. Likewise when the beans inside a runner-bean pod have swollen and will shortly be ready to burst the pod and drop to earth. Gardeners know how to fool the plants into continuing to produce for a longer period by “dead-heading” dying flowers and picking runner-bean pods when they are still very young.
THORNS, SPINES, POISONS, GAS – AND HELPERS
Among the many features that plants have developed are an impressive array of defences. Filamentous algae, with low mass investment and constantly renewing, probably did not need defences nor perhaps did the plants that first came ashore. Defence against what, after all? But later, as soon as animal life began to develop on land …..
Here in the north-west of Europe we are familiar with thorns and spines on the trunks and branches of the rose and briar, blackberry, gooseberry, gorse, blackthorn and hawthorn. It is not always on the trunks and branches that the sharp spikes are to be found, as we are reminded by the prickly leaves of the thistles and holly. Thorny and spiny defences are repeated around the world on other plants from acacias to cacti and many others. Thorns stab, rip and tear but spines lodge in the skin and continue to irritate, some forming sites of infection.
Well at least you’re safe among grass, right? Not necessarily, for example the dune builder grasses, marram or beach grass, can cut the skin of mammals moving through it. In other parts of the world they have aptly-named ‘sword’ and ‘saw’ grasses. Some of these cut with a thin edge but many with tiny hair-like spines growing on the underside of grass blades, called trichomes, defend against herbivorous invertebrates but may also cause “grass itch” in some people.
Mostly, these are a defence against grazing animals or protection against the theft of the plants’ fruits. Other plants have developed poisons, which they employ not only against mammal and bird grazers but also against insects such as caterpillars (as commented earlier) and locusts; examples in Ireland are the foxglove and the deadly nightshade or belladonna, a relative of the tomato and potato. Another is the hemlock, a relative of the carrot, parsley and angelica plants – even its sap can burn your skin. An invasive shrub or small tree, the cherry laurel, carries arsenic within its wood, leaves and berries and can be seen in many gardens, parks and growing wild around much of Wicklow.
But trees have also been observed to emit chemical compounds that attract the enemies of parasites or grazers feeding on the trees.
Poisons can be employed against competing plants too, as does the hydrangea, a shrub with lovely luxuriant flowers in your garden (or indeed in a public park in Howth) but a seriously invasive plant in the wild as it eliminates its competition and grows unchecked. It does this by a relationship with a bacteria around its roots that produces a poison to kill competing vegetation. However, the native pine also produces an allelopathy in its discarded needles, inhibiting the germination of other plant seeds and growth – it is not only the blocking of sunlight that keeps pine forests so free of undergrowth.
The onion carries an aroma warning that rough handling of the bulb will produce a gas attack on eyes and nasal passages, as known to any who have handled them in food preparation.
Plants employ some poisons continually but others selectively, as in ripening seeds (for example in the seed pods of the laburnum) or in sensitive growing tips (for example the fiddleheads or curled growing tips of bracken, toxic to grazers). The daffodil is a lovely plant and safe to handle but digging up the bulbs and mistaking them for wild onions can have fatal consequences for the eater. And as we have seen elsewhere, leaves can become poisonous as trees mobilise chemicals from tree to tree when under attack by caterpillars.
However, some plants welcome insects as protectors too, as for example with a species of ant that lives in some acacias and helps keep the tree free of pathogens.
When considering plant poisons we are reminded too of the stinging nettle, which introduces its defence to us in childhood, never to be forgotten. In North America, one would always remember a brush with its poison ivy. The Giant Hogweed, also a member of the carrot and parsley family but invasive to Ireland, causes a very painful rash following bare skin contact.
There are many localised wars going on out there.
FLAUNTING FLOWERS – AND FLIERS, SAILORS, ROLLERS AND HITCHHIKERS
Along with all their other innovations, plants evolved some very impressive ones in procreation, particularly in dispersing the next generation. Pollen, a fine powdery substance that is the equivalent of mammal sperm could be and was spread by the wind. The development of the flower and blossom brought in a partnership with animal pollinators to greater efficiency. Attracted by nectar and to some extent pollen, both insects and some birds visited male plants flaunting their flowers and unconsciously picked up pollen which they deposited at another flower they visited, thereby soon fertilising female flowers.
Flowers were developed in a huge variety of shapes and colours in order to attract pollinators — and then came smell. Some botanists speculate that scent was first used by some plants to discourage insects and grazers which, if true, is amazing enough. To then go on to develop scent to attract pollinators is a leap that staggers the imagination. Flowers and blossoms using smell are particularly noticeable at dusk and night, a time when flowers are hardly visible, when presumably they are visited by moths.
Early plants did not have seeds so the whole paraphernalia around them had to be developed from other existing parts with originally different functions (some of us could convert a bicycle, a machine for locomotion, into an electric power generator but still ….)
Behind the flowers of many species is a little node which when fertilised begins to swell and form a fruit, with the developing seeds inside — or single seed in the case of Prunus species, the plum family, for example. This is another amazing trick of the plant – it has produced attractive fruits, full of sugars when ripe, to attract animals (such as ourselves) to pick them and either discard the seeds as we eat the fruit or pass them through our gut to be deposited on earth — along with a handy dollop of manure. A botanist investigating the occurrence of isolated copses of trees on the grassy plains of the South American Pampas concluded that horses were eating the nuts of the parent trees some distance away then, as they travelled across the plains, at some point defecated with some intact nuts among their faeces: some years later – a grove of trees. Of course horses have only been in the Pampas for a few centuries and probably the other local grazers don’t eat saplings.
Nuts are also stored in different caches by some mammals and birds, for example here in Ireland by squirrels and magpies. They don’t always dig up all the stores later – perhaps they forget where some of them were – and in the spring, those nuts become saplings.
Well enough. But producing fruit and nuts is a lot of work and depends on the assistance of animals, especially mammals and birds, for dispersal. Some plants scorn to use them and instead employ the wind. Dandelions, thistles and many other plants send their seeds off on downy parachutes, often to land kilometres away. Some, like the sycamore, grow “wings” on their seeds which, when dry, spin away on the wind and not only that but when they strike mud are sometimes twisted by the wind on their “wing” to ‘screw’ the seed into the soil.
Many plants with pods, for example the legumes, will have their pods crack open when dry to “spill the beans” upon the soil. That is not good enough for the gorse or furze, the pods of which explode on a summer’s day, shooting the seeds away. One such day I sat among gorse bushes on Killiney Hill and was startled to hear what sounded like a weak pistol shot. Then another …. and another …. and all around me the bushes were shooting out their seeds, the lucky ones to create new bushlets (yes, I did just make up that word) the following year.
The casings of chestnuts, both edible and the ‘conker’ variety hit the ground, some cracking open as they do so and roll away from the tree. The casings of the edible ones are spiny, which no doubt afford the nuts inside some protection from being eaten (and trodden) until they are covered by fallen leaves or strike a root into the ground. Again, the lucky ones will become saplings and, enough sunlight (and goats) permitting, grow to become trees. The Mexican “jumping bean” rolls itself away from its parent, turning over and over, albeit slowly.
With fruit and nuts we saw plant offspring being cached or stowing away inside birds and mammals. But some hitch-hike on the outside too, like the burs that work their way into animal fur and into our woolen clothing. These are seed cases covered in tiny hooks, said to have been the inspiration for the invention of velcro fastenings in clothes. The cleaver or “sticky-back” may attach many of its small burs to a passing mammal, while the burdock, with its much larger burs, is more likely to hitch a ride in ones or twos. Tiny seeds of many grasses stick to wool, fur and hair too, especially when damp. But many other grasses with larger seeds, including cereals, grow “ears” with spikes attached to each seed and these too, when dry and ready to go, get picked up by the wool or fur of passing traffic.
The pines even use forest fires to spread seeds from inside their cones on the hot wind – each seed has a little vane around it to help it sail the wind. Sure, many will burn before they sail or blow into another fire – but some will survive. The alternative is just to burn.
The coconut, on the other hand, floats its fruit to distant shores – it is not for tourist brochures that the palms grew fringing tropical beaches. Falling coconuts roll away from the tree too – if they don’t hit some unfortunate large animal first. Many other plants use floods to populate different areas, often creating stronger banks or islands as their offspring grow, sometimes even changing the very course of a river or stream. The various willows and alders are adepts at this, as are many kinds of reeds and rushes.
The latter kind of colonisation may be by seeds but there are other methods too: severed branches or leaves that grow roots into water, uprooted saplings, tubers and bulbs. Bulbs, rhizomes and strings of tubers have been used by many plants to store food for offspring, nascent new plants hiding below or on the ground. Even when a field of potatoes is harvested, there are often tiny potatoes remaining that escaped the harvesting procedure – the following year, they may be seen, sprouting new plants.
Some plants are capable of employing all of the various methods of reproduction and distribution: seed, tuber, branch or leaf regeneration.
A somewhat similar method to strings of tubers – and possibly their actual origin – is the underground runner, like a root running just below and parallel to the surface, sending out shoots upwards and roots downwards at intervals, each of those becoming a new plant, a clone. Many grasses employ this procedure, some bunching close like the bamboo and others spreading away in different directions, as for example with the couch or scutch grass. The latter may be to the despair of the gardener, who however will use runners of the strawberry to grow new fruiting plants.
Grasses are a late and special kind of plant that can be grazed down to ground level and grow again, year after year. This provided a renewable food source for animals that could convert its leaves and seeds into sufficient energy – enter herds of goat and sheep, horse, donkey, zebra, deer, antelope, bison and cattle! And therefore enter their predators too, in particular the big cats, canines and – homo sapiens. She in turn would domesticate some of those species, including another predator as helper, the canine. That combination would change the world quite significantly and when homo sapiens learned to cultivate some of the grasses for their seeds, i.e cereals, well ……!
NB: Thanks to Oisín Breatnach for editing work (all subsequent errors etc are mine) and Osgur Breatnach for reminding me of the onion in a separate discussion.
REFERENCES & SOURCES
Native and invasive plants to Ireland: http://www.wildflowersofireland.net/
Deciding when to germinate: https://www.smithsonianmag.com/smart-news/seeds-use-tiny-brains-decide-when-germinate-180963625/
Tiny bristles in grass: https://www.quora.com/Why-does-grass-make-you-itchy
Ants protecting acacias from pathogens: http://www.messagetoeagle.com/acacia-tree-uses-ants-as-body-guards-and-rewards-them-with-shelter-and-food/
Plants inhibiting germination of competitors: https://en.wikipedia.org/wiki/Allelopathy
Poisonous effects of bracken on ruminants: https://link.springer.com/article/10.1007/s00580-018-2636-2
Trees summoning the predators on caterpillars: https://lt.org/publication/how-do-forest-trees-defend-themselves-against-insects-under-natural-conditions-and