Ancestors of Land Plants

Skyrail Nature Diary: October 2014


This month we will be looking at the ancestors of land plants and some modern, primitive descendants that resemble them. Algae are the obvious ancestors of the land plants. The question is which ones? Thanks to DNA research, it’s now generally known that algae are a diverse group of primitive organisms with little or no relationship to one another. Most algae are microscopic. Only three groups of algae have members large enough to be seen easily with the naked eye. These are the green algae, brown algae and the red algae. Only the green algae has freshwater species. In the ocean, the three groups have different distributions due to different light collecting pigments. In water, colours are absorbed as you descend into the depths, blue always the first to disappear. This is why most algae and animals appear red 30 metres below the surface. Green algae has similar pigments to land animals designed to absorb blue and green light. Brown algae has brownish pigments designed to absorb light in the midrange (yellow and orange). Red algae is always found deepest and are specialised in absorbing red and far red light. Some of the red algae are blue green, as are their ancestors the blue-green algae. 

Stoneworts (Charales) have large, visible thalli (algal body) growing up to 120 cm long. They are branched, multicellular and grow in fresh water. They are called stoneworts because the plants can become encrusted in lime (calcium carbonate) after some time. The "stem" is actually a central stalk consisting of giant cells with many nuclei per cell. They are unique in having a whorl of small branchlets at each node, giving them a superficial resemblance to the horsetails. In these whorls it is possible to see the phenomenon of cytoplasmic streaming. In fact the streaming in stoneworts is the fastest recorded of any cells. Cytoplasmic streaming is caused by the microfilaments found inside the cell. Despite resembling green algae, these plants produce female organs encased in an envelope of sterile cells, something normally only seen in land plants. There are about 400 species of stoneworts worldwide.

Mosses are the most primitive of the modern land plants. Most biologists agree that they are a completely separate group of land plants not related to the rest. They have no vascular tissues and have a slightly different life cycle to the vascular plants. Whereas vascular plants favour the asexual stage, reducing the sexual stage, the mosses favour the sexual stage. The asexual stage grows directly on its parent as a mild parasite and usually has no chlorophyll with the exception of the hornworts. Hornworts have now been shown to be closely related to the vascular plants and thus not real mosses. The male mosses are usually dwarfs. There are two types of mosses, the liverworts and the true mosses. Like the hornworts, the liverworts have now been shown to be closer to the sister groups hornworts and vascular plants. Either way, all mosses require plenty of moisture, related or not. Interestingly some tiny mosses grow on the surfaces of leaves together with fungi and microalgae. True mosses appear to have true leaves but lack any vascular tissues. Liverworts look like green, uneven sheets.

The earliest land plants had very little in the way of supporting tissues in their cells and consequently, were all fairly small. They also had no leafy structures or real roots. The earliest known fossil species of these land plants is Cooksonia, discovered in Britain. They are believed to have grown near the water’s edge and produced flat, round spore bearing structures on the ends of slender stalks. They probably had chlorophyll in the stalks as there is no evidence of anything resembling leaves on the tiny plants. The earliest fossil plants with the beginnings of possible leaves we know of are Rhynia, named after the Rhynie district in Scotland where they were discovered. They were very similar to Cooksonia with but are slightly larger and branch more extensively. All the following plants produced simple spores and had separate life stages until the first seeds plants turned up, the Seed Ferns. These plants looked like tree ferns but had true trunks and produced seeds. Seeds incorporate both stages of the life cycle inside, the asexual stage being the dominant stage. The sexual stage gradually ended up being reduced to a tiny cluster of cells in the cycads and the conifers and to one single cell with eight nuclei in the flowering plants, totally dependent on the asexual stage.

by Tore Lien Linde