Concepts: Pteridophytes

3.1 The first vascular plants

Pteridophytes are the horsetails and the ferns. They grow in cool, damp, shaded places, although some do well in sandy soil. They are used as medicines, as soil binders and as ornamental plants. Their place in evolution is the interesting part: they are the earliest land plants with vascular tissue, the xylem and the phloem.

The main plant body is the sporophyte, and it is differentiated into a true root, a stem and leaves. These organs carry well-differentiated vascular tissue. That tissue is the real novelty of the group: it moves water and food over distance, so a tall upright sporophyte can keep all its parts supplied. The leaves are small in some genera, as in Selaginella, and large in others, as in most ferns. The sporophyte bears sporangia, and each sporangium is held above a leaf-like appendage called a sporophyll. In some genera the sporophylls pack together into compact structures called strobili or cones, as in Selaginella and Equisetum. Spore mother cells inside the sporangia divide by meiosis and make spores.

A spore grows into a small, many-celled, free-living gametophyte called the prothallus. The prothallus is mostly photosynthetic and thalloid, and it needs cool, damp shade. That narrow requirement, together with the need for water at fertilisation, limits where pteridophytes can spread. The prothallus bears antheridia and archegonia. Water carries the antherozoids released by the antheridia to the mouth of the archegonium, and one fuses with the egg. The zygote grows into a well-differentiated sporophyte, which becomes the dominant phase.

3.1.1 Homospory and heterospory

Most pteridophytes produce spores of one kind, and such plants are homosporous. Selaginella and Salvinia produce two kinds, larger megaspores and smaller microspores, and such plants are heterosporous. The two kinds of spore grow into female and male gametophytes. In these heterosporous plants the female gametophyte stays on the parent sporophyte for some time, and the young embryo develops inside it. That arrangement points towards the seed habit, and the chapter calls it a precursor of that habit, which is a step of real importance in evolution.

3.1.2 The four classes

Pteridophytes are divided into four classes. Psilopsida holds Psilotum; Lycopsida holds Selaginella and Lycopodium; Sphenopsida holds Equisetum; and Pteropsida holds the ferns such as Dryopteris, Pteris and Adiantum. The classes are separated largely by the form and arrangement of the sporophyte.

The examples are the quickest way to hold the four names, because each class is tied to its own familiar genus.

3.1.3 What limits the pteridophytes

The prothallus is the weak point of the group. It is small and thalloid, it has no vascular tissue, and it can hold very little water. It must grow in cool damp shade, and the antherozoids need a film of water before they can reach the archegonia. Both conditions have to be met at the same time, so living pteridophytes are tied to narrow, moist patches of ground. The sporophyte itself can be fairly large and well supplied with vascular tissue, yet the group cannot spread far from its damp footholds. A prothallus that dries out dies, and the eggs inside its archegonia are lost with it. Ferns do best on a shaded forest floor, and a few species manage on sandy soil.

3.1.4 Uses of pteridophytes

Pteridophytes are used for medicinal purposes, and they serve as soil binders. Several ferns with large fronds are grown as ornamentals, and because their roots hold the soil, they are useful on slopes that would otherwise wash away. In a garden the fronds give the shade-loving group its value, and the same roots quietly bind the ground under the plant.

3.1.5 Practice from the NCERT exercises, questions 7 and 9

Exercise 9, part three. A homosporous pteridophyte makes one kind of spore, and its gametophyte is free-living on damp soil. A heterosporous pteridophyte makes microspores and megaspores, and the female gametophyte from a megaspore stays on the parent sporophyte while the embryo develops inside it.

Exercise 7. Heterospory is the production of two kinds of spores, microspores and megaspores, by one plant. Its significance lies in what follows from it. The megaspore grows into a female gametophyte that stays on the parent plant, the microspore grows into a male gametophyte, and fertilisation can happen while the female gametophyte is still attached. The young embryo then develops inside that gametophyte on the parent plant. This retention of the embryo is the step that leads towards the seed habit, and Selaginella and Salvinia are the two examples named in the chapter.