Classification

Vascular

The earliest vascular plants have differentiated parts and are seedless (cryptogams). They have tissues called xylem that transport water up and food out to tissues, called phloem. Together the xylem and the phloem are called vascular tissue. Vascular plants have roots, stems, and leaves. The earliest include club mosses horsetails ferns (pterophyta) most numerous of this type of plant These all grow in boggy conditions often on rocks/silt/water edges. They did not need soil. Whereas the next step was on to the land, and to achieve that the plants needed to cope with drier conditions. We are examining the place and period when primitive vascular plants evolved to become seed plants, and that seems to have happened when they tried to live on land. They need to adapt to help them survive. They are covered with a waxy layer, or cuticle that holds in water. They also have stomata, or pores that help them take in and let out gasses like carbon dioxide and oxygen. Their roots take up water and nutrients from the soil and anchor them to the soil. Stems move water and nutrients to the plant's leaves and the leaves capture the sunlight the plant needs for photosynthesis. (from Nature Works). There are late Devonian free-sporing plants such as Archaeopteris, which had secondary vascular tissue that produced woodland had formed forests of tall trees. Also by late Devonian, Elkinsia, an early seed fern, had evolved seeds.[9] Evolutionary innovation continued into the Carboniferous and still goes on. So we are talking about the key part of plant evolution – on to land, at the same time as the volcanic ash is about.

Higher

The next step was the evolution of Higher plants, which form a large group of plants (over 300,000 [3]). They are defined as those land plants that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. It is generally agree lignin was crucial to plant colonisation of the land. It is a very hard-wearing molecule stronger than cellulose, and enables trees to grow hundreds of feet high. It builds strong walls to carry water, which ender pressure can travel up a long way. Copper – also in that volcanic dust, activates enzymes involved in the formation of lignin. Manganese, also in volcanic dust, is required by some enzymes to break down lignin on the forest floor. While the phylogeny of plants is pretty well agreed, there doesn’t seem to be much on how and when the vascular plants came onto land. Although all are clear they did. Obviously there wouldn’t be just one time, but this particular period does seem to fit the bill very well.

The success of these higher plants (gymnosperms and angiosperms) may be attributed to:

• development of an extensive root system and for an extensive root system you need either volcanic dust or soil.

• an efficient vascular system (xylem and phloem) to carry the water from the roots to the top

• reproductive structure in which the gametophyte is protected inside sporophyte tissue (the seed)

Seed plants include: naked-seed plants (such as conifers), known as gymnosperms and flowering plants - the angiosperms.

We are talking about the evolution from tracheophytes to gymnosperms and on to angiosperms. Without soil, there would be no seed plants.