21.1 Early plant life

Introduction

I observed on most collected stones the imprints of innumerable plant fragments which were so different from those which are growing in the Lyonnais, in the nearby provinces, and even in the rest of France, that I felt like collecting plants in a new world... The number of these leaves, the way they separated easily, and the great variety of plants whose imprints I saw, appeared to me just as many volumes of botany representing in the same quarry the oldest library of the world.

The kingdom Plantae constitutes a large and varied group of organisms, which have been on the planet for a very long time. There are more than 300,000 species of catalogued plants, including the fossil plants that de Jussieu references in the epigraph above. Of these, more than 260,000 are seed plants. Mosses, ferns, conifers, and flowering plants are all members of the plant kingdom. While there is some disagreement about the relationships between Chlorophytes, Charophytes, and Plantae, there are several unique characteristics which these groups share. Only green algae and plants use chlorophyll a and b plus carotene in a particular ratio. They share the trait of cellulose-rich cell walls, and there is strong molecular support for their close relationship.

The ancestors of the green algae became photosynthetic by engulfing a green, photosynthetic bacterium about 1.65 billion years ago. This captured bacterium evolved into a chloroplast. That algal line evolved into the Charophytes, bryophytes, seedless vascular plants, gymnosperms, and angiosperms.

Several other groups of eukaryotes have common names that include ‘algae.’ In the latest classification red algae are included in Archaeplastida, while brown algae and golden algae are in a separate supergroup. In contrast to the green algae, red, golden, and brown algae all became photosynthetic by secondary, or even tertiary, endosymbiotic events. In other words, the cells that evolved into red, brown, or golden algae all engulfed cells that had already engulfed a photosynthetic bacterium. These algae are also photosynthetic autotrophs, but they did not diversify to the same extent as the Charophytes, nor did they colonize land.

Plant adaptations to life on land

In order for plants to invade land, they had to contend with several challenges in the terrestrial environment. Water has been described as “the stuff of life.” The cell’s interior is a watery soup: in this medium, most small molecules can dissolve and diffuse rapidly, and the majority of the chemical reactions of metabolism take place. The first challenge, Desiccation , or drying out, is a constant danger for an organism exposed to air. Even when parts of a plant are close to a source of water, the aerial structures are likely to dry out. Second, Water also provides buoyancy to organisms. On land, plants need to develop structural support in a medium that does not give the same support as water. The organism is also subject to bombardment by mutagenic radiation, because air does not filter out ultraviolet rays of sunlight like water does. Additionally, the male gametes must reach the female gametes using new strategies, because swimming is no longer possible. Lastly, both gametes and zygotes must be protected from desiccation. The successful land plants developed strategies to deal with all of these challenges. Not all adaptations appeared at once. Some species never moved very far from the aquatic environment, whereas others went on to conquer the driest environments on Earth.