Part I: Expedition to the Unknown
(Reading time: 5 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 probably 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 and also bow to high wind. 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 already 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.