I recently spent some time at the beautiful Lincoln Park Conservatory. It is very hot and humid in the greenhouse, which may not have been the most obvious choice on a day that was already 88º outside. Nevertheless, I thought it would be a good way to talk about the evolution of plants. My specialty in biology is animals, but I have always loved the story of plant evolution. Like all other major groups of life on Earth, plant life began in the water. Early plants were very reliant on water, but plants became less and less dependent on water as they evolved. In the water, life is easy. Dehydration is not a problem. Nutrients can be absorbed directly out of the water into the cells. The water will carry sperm for reproduction and disperse the offspring. Life in the water is good, but there was a lot of space to grow on land.
One of the first types of plants to live on land were the liverworts. They are very small, with leaves that lie almost flat on the ground. Liverworts have no ability to draw water up out of the ground, as later plants are able to do. As a result, they cannot grow very tall and must be damp all of the time. They cannot survive or reproduce without being wet.
Like liverworts, moss also cannot draw water up into their bodies. Are able to grow a little bit taller than the liverworts because they grow in dense mats that can trap water between individual plants. This allows them to grow up to about four inches tall.
Mosses do not dry out as easily as liverworts, but they do rely on water for reproduction. The male sperm must swim through the water to find a female plant.
Club moss are sometimes called “ground pines” because they can resemble pine trees, but they are neither pines nor moss. Modern club moss usually only grow to be a few inches tall, but during the Carboniferous period, when they were the dominant land plant, they grew as tall as modern trees.
Club moss are one of the first type of plants to have vascular tissue, which lets them draw water from the ground up into their bodies. This adaptation is of unparalleled importance for plants on land. For this reason, club moss were one of the first types of plants to be able to grow more than a couple of inches tall. Without vascular tissue, a plant more than an inch or two has no way of getting water to the upper part of the plant. Plants don’t need very much to live, but access to sufficient sunlight is one of their main requirements. When all of the plants around you are only two inches tall, a plant that can grow to be several feet tall or taller has an enormous advantage when it comes to getting sunlight. You can grow taller than your neighbors and spread out to get all the sun you want.
Coal is made of fossilized plants from the carboniferous period. The majority of coal is made up of ferns and club moss. Sometimes coal preserves the structures of the plants it was made from and we can use the coal fossils to learn about ancient plants.
Evolutionarily speaking, ferns are slightly newer than the club moss. Like the club moss, ferns are have vascular tissue (so does everything else from here on).
Compared to most other plants, ferns grow sideways. The stem lies horizontally underground, and the fronds grow up out of the ground from it. When you see a cluster of fronds sticking up together, they are usually from the same plant.
Structures on the underside of the fronds, called “sori,” contain spores. These capsules break open, releasing the spores, and new ferns grow where the spores land.
Cycads look superficially like palms, pineapples or yuccas, but is not closely related to any of them. They were one of the dominant types of plants during the mesozoic era — the age of the dinosaurs.
Cycads were among the first plants to use pollen in reproduction. Pollen is produced by the male structures on plants, and is responsible for carrying sperm to the ovule in the female structures on other plants. Unlike the earlier plants, which require water for the sperm to swim through, pollen is carried by the wind. This is great for plants that live away from water and want to be able to reproduce with individuals that are far away. The problem is that it is fairly inefficient. Plants whose pollen is carried by the wind need to produce vast quantities of the stuff in order for some of it to get to other plants. I grew up in New Hampshire, where there a lot of white pine trees (which are not cycads, but also reproduce with wind-borne pollen). I got up many a morning to find my car completely covered in yellow pollen. All of that pollen that didn’t end up on the right part of the female plants is wasted energy.
Cycads were also among the first plants to have seeds, instead of spores like the older plants. Spores are fine, but they cannot travel over long distances or lie dormant for a more opportune time to sprout. A seed contains the plant embryo as well as nutrients to keep it alive for months or years. If a spore happens to land on the back of a bird on its way to the other side of the country, the embryo inside may not survive the trip because its mother didn’t pack it lunch. An embryo inside a seed will survive the same journey because it is surrounded in an oil-rich substance called “endosperm.” When we eat nuts, it is the endosperm that we are after.
Flowering plants became the dominant plants of the world during the late mesozoic, and today account for the majority of plant species.There are over a quarter million living species of flowering plants, compared to only about 12,000 species of fern, and fewer than 10,000 species of liverwort.
The flowering plants have two evolutionary advancements that allowed them to be so successful: flowers and fruit. These allow plants to solve two big problems in the area of reproduction.
Flowers represent an exchange of goods and services between plants and animals. Big, colorful, aromatic flowers are nature’s equivalent of an “eat here” sign. Flowers produce sugar-rich nectar that animals like ants, butterflies, birds, and bats like to eat. While these “pollinators” are eating the nectar, they get covered in pollen. When they go to the next flower, they drop some pollen off and pick up some more. This results in animals carrying pollen directly from one plant to the next, with very little waste. Remember all that energy that earlier plants wasted trying to pollinate my car? Flowers allow plants to use their energy more effectively. The energy this saves over relying on wind pollination is part of why flowering plants are so successful evolutionarily.
Spreading seeds is another problem for plants. If seeds just fall off the parent plant and onto the ground, some will roll away or get kicked away. The others will sprout right next to their parent and compete for the same nutrients and light. This will reduce the success of both the parent and the offspring. Some (but not all) flowering plants produce fruit to solve this problem.
Fruit is a sugar-rich substance that is easy to get eat, which surrounds the seed, which contains an oil-rich substance that is protected by a hard shell. In human terms, the plant “wants” you to eat the fruit, but does not “want” you to eat the seed. If you eat the seed, you are eating the tree’s offspring. If you (or another animal) eat the fruit, there is a good chance that you will swallow the seeds by accident. The hard shell protects it from being broken in your mouth or digested in your stomach. After eating the fruit, you (or whatever animal) will walk or fly away and eventually deposit the seeds far away in a nutrient-rich pile of fertilizer.
Flowering plants are better at life on land than any other plants. They can draw water and nutrients out of the soil through their roots and vascular tissue, and they are very good at reproducing and spreading their offspring without the aid of water. This is why they are beating all of the other plants.