Plant Science Portal 
First Vascular Land Plants (1)
 Plant Evolution Tour: Part III of XV

Before totally colonising the land, plants moved into the intertidal zones. Like modern plants today, they had developed cuticles and stomata and due to the multicellular structure of these organisms, a very basic vascular system.

Living on land meant that plants needed to develop means by which to minimise water loss and hence a cuticle was essential. The cuticle is a waxy layer which covers most plants and prevents water loss directly through cell walls. Stomatal pores (singular stoma) also quickly developed to regulate water loss. And through necessity, plants developed supporting cells to compensate for the absence of water and to strengthen them against winds and rain.

By the Devonian (c.408 Ma) low-growing vascular plants had evolved on most continents, and there are many fossils which date from this time to indicate this. Interestingly, many of these are macrofossils of complete stems rather than microfossils of individual cells.
Cooksonia pertoni
Fossil of Cooksonia pertoni. (Photo by Hans Steur, used with permission).
As passive absorption of water was no longer possible, rhizoids and then roots evolved to anchor plants and to absorb water and nutrients. Once the roots were able to absorb nutrients and water, conducting tissues developed to move these through the body of the plant. All this lead to the formation of vascular plants - those which had specialised woody vessels called xylem to conduct water.

One of the most significant early terrestrial plant species are a group classified as Cooksonia.

Cooksonia was telomous - it had no leaves. The genus first appears in the fossil record during the Silurian (428 Ma) and a number of substantial fossils have been unearthed in Ireland. Cooksonia was simple in structure; dichotomous branches formed the body of the plant, and each was terminated with sporangia (from where spores are released). Willis & McElwain (2002) suggest that these species grew no more than 6.5cm in height and would have naturally occurred in swampy locations. The spores which these plants released were dispersed by wind.

Another significant early species was Aglaophyton major. Fossils dating back to 400 Ma have been found in the Rhynie Chert of Scotland, UK.

Aglaophyton is considered to be more advanced than Cooksonia because unlike the latter, it had branched stems which grew to 20cm in height and which were morphologically more complex. Prostrate rhizomes and rhizoids provided anchorage in the ground.

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