How did plants invade the land?
Any living creature had to overcome a number of problems in moving from the water to the land. These include protection against desication, support, fluid and nutrient transport, gas exchange, ultraviolet radiation, and reproduction. The fin-to-limb post looked at how animals overcame those obstacles, but animals had to have something to provide an energy source before they could even begin to make the leap to land. That means that photosynthesizing organisms had to move onto land before animals could even get a start. So how and when did they do it?
Algae have been around for a long time – something like 2.1 billion years. Photosynthesizing bacteria are even older, and either could have formed thin mats where wave action or stream turbulence kept the ground wet. I’ve put photosynthesizing slimes in the wave zone on Mirror, one of the planets I imagined for Homecoming. Such thin mats or slimes, however, would be been dependent on being kept wet.
The first appearance of cuticle, the membrane that acts as a skin for plants and keeps them from drying out, was in the Ordovician, around 450 million years ago. Once you have a cuticle which prevents the interior of a cell mass from drying out, you also need some way of allowing carbon dioxide into the cells and oxygen out so stomata, the pores plants breathe through, are probably about the same age.
Early plants began to lift their fruiting bodies above the ground, but not far. They also had branching, cuticles, stomata, and photosynthesizing stems, but no leaves. Ultraviolet protection was probably a combination of pigments and enough oxygen in the atmosphere that an ozone layer formed. Gradually plants evolved mechanisms that transported nutrients and fluids via hollow cells that combined to form tubes (vascular system.) They also needed a system of roots to hold them upright.
The first plants did not have soil as we know it. A certain amount of mechanical weathering would have broken the rock down into silt or sand, but today’s soil is largely the result of chemical weathering (in which plants assist) and decayed plant matter.
Support systems developed slowly. The plants gradually began to show secondary growth (stems widening with age, instead of just getting longer) and the ability to synthesize cellulose from the sugars they produced via photosynthesis. By the time the first amphibians crawled out of the water, tall trees had developed. Why? Most likely competition for light.
Plants were not, however, a good food source. Even without the toxins that plants of today have evolved to avoid being eaten, neither cuticle nor cellulose is easy to digest. The herbivorous animals of today have a long history of adaptation to plant eating, but even so a large part of their (and our) digestion of plants is actually carried out by microorganisms.
The millipedes and other invertebrates that preceded the amphibians onto land were mostly detritus feeders, relying on bacteria and fungi to decompose the plant matter enough they could digest it, or carnivores that ate the detritus feeders. The fact that so many of the plants of the Carboniferous wound up as coal may even be due in part to the fact that few animals could eat them. Early amphibians may well have obtained most of their nourishment from insects and from the water.
Although many of the early plants reproduced by spores rather than seeds (as do mosses and ferns today) they had evolved seeds by the Devonian (around 380 million years ago.) Our amphibian ancestors would have found forests that included trees resembling today’s conifers and ginkgo. Flowers, however, came much later.
This is the last summary of the OLLI classes on major evolutionary transitions. Next Friday’s post will be on some other aspect of science.