25.1 Early Plant Life
Land plants acquired traits that made it possible to colonize land and survive out of the water. All land plants share the following characteristics: alternation of generations, with the haploid plant called a gametophyte, and the diploid plant called a sporophyte; formation of haploid spores in a sporangium; formation of gametes in a gametangium; protection of the embryo; and an apical meristem. Vascular tissues, roots, leaves, cuticle cover, and a tough outer layer that protects the spores contributed to the adaptation of plants to dry land. Land plants appeared about 500 million years ago in the Ordovician period.
25.2 Green Algae: Precursors of Land Plants
Charophytes share more traits with land plants than do other algae, according to structural features and DNA analysis. Within the charophytes, the Charales, the Coleochaetales, and the Zygnematales have been each considered as sharing the closest common ancestry with the land plants. Charophytes form sporopollenin and precursors of lignin, phragmoplasts, and have flagellated sperm. They do not exhibit alternation of generations.
Seedless non-vascular plants are small, having the gametophyte as the dominant stage of the lifecycle. Without a vascular system and roots, they absorb water and nutrients on all their exposed surfaces. Collectively known as bryophytes, the three main groups include the liverworts, the hornworts, and the mosses. Liverworts are the most primitive plants and are closely related to the first land plants. Hornworts developed stomata and possess a single chloroplast per cell. Mosses have simple conductive cells and are attached to the substrate by rhizoids. They colonize harsh habitats and can regain moisture after drying out. The moss sporangium is a complex structure that allows release of spores away from the parent plant.
25.4 Seedless Vascular Plants
The seedless vascular plants show several features important to living on land: vascular tissue, roots, and leaves. Vascular systems consist of xylem tissue, which transports water and minerals, and phloem tissue, which transports sugars and proteins. With the development of the vascular system, leaves appeared to act as large photosynthetic organs, and roots to access water from the ground. Small uncomplicated leaves are termed microphylls. Large leaves with vein patterns are termed megaphylls. Modified leaves that bear sporangia are called sporophylls. Some sporophylls are arranged in cone structures called strobili.
The support and conductive properties of vascular tissues have allowed the sporophyte generation of vascular plants to become increasingly dominant. The seedless vascular plants include club mosses, which are the most primitive; whisk ferns, which lost leaves and roots by reductive evolution; and horsetails and ferns. Ferns are the most advanced group of seedless vascular plants. They are distinguished by large leaves called fronds and small sporangia-containing structures called sori, which are found on the underside of the fronds.
Both mosses and ferns play an essential role in the balance of the ecosystems. Mosses are pioneering species that colonize bare or devastated environments and make it possible for succession to occur. They contribute to the enrichment of the soil and provide shelter and nutrients for animals in hostile environments. Mosses are important biological indicators of environmental pollution. Ferns are important for providing natural habitats, as soil stabilizers, and as decorative plants. Both mosses and ferns are part of traditional medical practice. In addition to culinary, medical, and decorative purposes, mosses and ferns can be used as fuels, and ancient seedless plants were important contributors to the fossil fuel deposits that we now use as an energy resource.