A maple tree is clearly very different from a moss, yet over a long period of time, and step by step, early moss-like plants evolved to trees, including maples. The fossil record tells us about some of these steps, and experts have agreed upon the probable steps where the fossils don’t tell us. The story centers on solving problems involved with the invasion of land from the sea or fresh water.
Hundreds of millions of years ago, green algae began to colonize land. They already had the mechanisms of photosynthesis: using light to combine carbon dioxide and water into nutritious carbohydrates. And they already had sexual reproduction, which produces new combinations of genes every generation and thus the variation necessary for evolutionary change.
Experts say, and there are hints in the fossil record, that the very early land plants formed associations with fungi that provided nutrients, a mutualistic association that almost all land plants have maintained. So those early terrestrial plants were off to a good start.
For sexual reproduction, however, those early plants were entirely dependent on water. Eggs and sperm were released into water, where sperm could swim a few centimeters to find a drifting egg to fertilize. That’s OK for plants living in water or even in damp soil, but it won’t work on dry land.
The first land plants are thought to have been more or less moss-like — small plants growing close to the ground. If conditions were not wet enough, these early plants had to wait for sex until conditions improved. Although most modern mosses still have to wait, as did the early plants, a few are reported to use springtails, mites or flies to transfer sperm to eggs, but few such animals were present in the early days of the invasion of land.
But even if sperm could swim to an egg, the resulting zygote would be still on its own in a potentially dangerous environment. Somewhere along the line, although sperm was released and still had to swim, eggs began to be retained in special structures on the leaves of the parent plant. Fertilization then occurred within these special structures and the zygote received both protection and some nutrition during development.
In mosses, that zygote stays on its mother and grows into a new individual (called a sporophyte) that looks different from its mother and eventually produces spores. Spores are single cells inside a tough coat that disperse on the wind, and, if they land in a good spot, grow into new mosses (called gametophytes, because they produce gametes). Thus, the generations alternate between gamete production and spore production.
There was still the problem of needing water for the sperm to swim to an egg. The fossil record is poor at this point, but clearly, at some point, moss-like plants began to produce two kinds of spores on their sporophytes: small, male spores with sperm, and big, female spores with eggs.
The big spores did not disperse but were retained on their mother sporophyte’s leaves, where they received protection and nutrition. The small spores ultimately developed more protective covers; they traveled on the wind and found special landing places near the female spores, where they could fertilize the eggs.
In some cases, that special landing place is a droplet of fluid, close to the egg, that engulfs the arriving male spore (if it belongs to the same species as the egg) and pulls it in. That is how pollination came about. The big, female spores began to stock nutritional material inside, for the growth of the embryo, and that was the beginning of the evolution of seeds.
The early land plants (and modern mosses) were often at risk of desiccation. Protection from drying out came with an impervious cuticle over the outer surfaces. But complete imperviousness would not only prevent water loss but also prevent carbon dioxide (for photosynthesis) from entering. Conveniently, surface pores called stomata, which can open and close, allow the entrance of carbon dioxide and help control water loss.
Mosses don’t have a very good system for transporting water from one part of the plant to another. But some of those early land plants developed a vascular system for water transport.
This is built from lined-up, hollowed-out cells with reinforced cell walls, called xylem; concentric bundles of xylem became known to us as wood. The resulting channels connect to the stomata. When a stomate is open, water vapor is lost faster than carbon dioxide comes in. This is called transpiration (in parallel with evaporation from a surface).
Because water molecules in the vascular channels cling together, transpiration from stomata pulls columns of water up through the channels. No energy expenditure needed, except to open or close stomata. These channels deliver water to leaves and other parts of a plant. The walls of the channels are reinforced to prevent collapse.
The early plants did not have true roots, just small anchoring fibers. True roots did not develop until there was a vascular system to deliver water to them. Conversely, without roots, the nascent vascular system could not draw much water from the soil. But, somehow, early vascular plants did develop roots of some sort, which not only anchored the plant to the soil, but also transferred soil water and nutrients to the plant above and to the roots themselves.
Development of a vascular system happened in sporophytes, not in gametophytes, which initially needed to stay small for sexual reproduction (but later began to stay on the sporophyte, as noted above). Those stiffened channels also provide mechanical support, so vascular plants could grow tall. Being tall is often an advantage when competing for light.
The radical remodeling of cells needed for the development of a vascular system in sporophytes may have been facilitated by the fact that sporophytes have two sets of chromosomes (compared to gametophytes and gametes with just one). That means deleterious mutations in one set of chromosomes could be masked by normal genes on the other set.
Thus, we arrive at a hypothetical ancestral plant, with some kind of pollination system that does not depend on water, a primitive seed with nutrition for an embryo, a vascular system, roots and the ability to grow tall, with mutualistic fungal associations.
The stage is set for the evolution of seed plants. There are basically two kinds of seed plants: all the flowering plants, including maples, oaks, shooting stars and lupines, whose seeds are enclosed in layers of maternal tissue, and the conifers and their relatives, whose seeds are borne on the surface of leaves or scales.
The evolution of these two branches of the plant evolutionary tree makes another complex story.
• Mary F. Willson is a retired professor of ecology. “On The Trails” is a weekly column that appears every Friday. Her essays can be found online at onthetrailsjuneau.wordpress.com.