21.3 Seedless vascular plants  (Page 4/5)

Lycophytes follow the pattern of alternation of generations seen in the bryophytes, except that the sporophyte is the major stage of the lifecycle. The gametophytes do not depend on the sporophyte for nutrients. Some gametophytes develop underground and form mycorrhizal associations with fungi. In club mosses, the sporophyte gives rise to sporophylls arranged in strobili, cone-like structures that give the class its name. Lycophytes can be homosporous or heterosporous.

Phylum monilophyta: class equisetopsida (horsetails)

Horsetails, whisk ferns and ferns belong to the phylum Monilophyta, with horsetails placed in the Class Equisetopsida. The modern genus Equisetum contains the only survivors of a large and diverse group of horsetails known as the Arthrophyta, which produced large trees and entire swamp forests during the Carboniferous. The plants are usually found in damp environments and marshes ( [link] ).

The stem of a horsetail is characterized by the presence of joints or nodes, hence the name Arthrophyta (arthro- = "joint"; -phyta = "plant"). Leaves and branches come out as whorls from the evenly spaced joints. The needle-shaped leaves do not contribute greatly to photosynthesis, the majority of which takes place in the green stem ( [link] ).

Silica collects in the epidermal cells, contributing to the stiffness of horsetail plants. Underground stems known as rhizomes anchor the plants to the ground. Modern-day horsetails are homosporous and an individual produces both male and female gametes.

Phylum monilophyta: class psilotopsida (whisk ferns)

While most ferns form large leaves and branching roots, the whisk ferns , Class Psilotopsida, lack both roots and leaves. Photosynthesis takes place in their green stems, and small yellow knobs form at the tip of the branch stem and contain the sporangia.

Phylum monilophyta: class psilotopsida (ferns)

Ferns are the most readily recognizable seedless vascular plants and are considered the most advanced seedless vascular plants because they display characteristics commonly observed in seed plants. More than 20,000 species of ferns live in environments ranging from tropics to temperate forests. Although some species survive in dry environments, most ferns are restricted to moist, shaded places. Ferns made their appearance in the fossil record during the Devonian period and expanded during the Carboniferous.

The dominant stage of the lifecycle of a fern is the sporophyte, which consists of large compound leaves called fronds. Fronds fulfill a double role; they are photosynthetic organs that also carry reproductive organs. The stem may be buried underground as a rhizome, from which adventitious roots grow to absorb water and nutrients from the soil; or, they may grow above ground as a trunk in tree ferns ( [link] ). Adventitious organs are those that grow in unusual places, such as roots growing from the side of a stem.