PTERIDOPHYTA, one of the groups comprising the ferns and their allies, of the second great division of plants, the Arche goniatae, the other being the Bryophyta (q.v.). The Pteridophyta thus share with the Bryophyta a middle position between the essentially aquatic Thallophytes and the essentially terrestrial Spermatophytes. Much of their special interest centres round that fact. They include plants well represented at the present day; but many already existed in the early land-vegetation of the Devonian period, now known only as fossils ; they appeared in greater profusion as fossils of the coal, and many of these early types continued on into the Mesozoic age while their correlatives are included in the flora of the present day. Thus their geological history supports the conclusion that they take a middle position in the evolutionary progression of plant-life, in which a transition from life in water to life on land was a striking incident.
Hofmeister first showed that there is essential similarity under lying the life-histories of mosses and ferns, and that the same scheme, in modified form, extends to the seed-plants also. Since this is so, an account of the Pteridophyta may fitly be introduced by a brief record of the life-history of a fern, as an example of the Archegoniatae generally, and of the Pteridophyta in particular. There are two periods in each normally completed life-cycle of these plants when the individual is represented by a single cell; and these punctuate the limits between two distinct bodily phases, or generations as they are called. One of these is the leafy fern plant which every one knows ; the other is a small green scale-like body, delicate in texture, called the prothallus, which escapes the observation of most people, though actually common enough.
The former is sexless, but bears spores in large numbers; it is the sporophyte generation. The latter is the sexual generation, and, since it produces gametes, it is called the gametophyte.
Life-history of a Fern.—The fern plant varies in size from a minute herb to a tree-like body, 6o or even Soft. in height. It consists of a stem bearing characteristic leaves, usually of large size and delicate outline; the shoot thus constituted is at tached by roots to the soil, the whole being traversed by conducting tracts. Since the
green leaves serve a nutritional function, the plant is able to subsist as a perennial land-plant (fig. I). On the leaves the sori are borne, of various form and position. In the hart's tongue or the common shield fern they appear as dense groups of brown sporangia seated on the lower surface, and covered while young by a membranous in dusium. Each sporangium is a stalked cap sule containing numerous minute, dry and dusty spores, which are violently ejected when ripe, and each is then liable to be carried away individually by the breeze. The spores are unicellular propagative organs.
Each spore germinating on moist soil may grow into a prothallus or gameto phyte (fig. 2), which never grows large, though being green it is physiologically independent. It bears the sexual organs or gametangia, usually on its lower surface (fig. 2, A). Near its base are the male gametangia or antheridia, which when ripe consist each of a protective wall of cells surrounding numerous spermatocytes. When bathed by external water (rain or dew) the wall ruptures, and each spermatocyte emits a single spiral spermatozoid, which moves in the water by lashing cilia (fig. 3). Near the indented apex of the prothallus the female gametangia or archegonia are formed (fig. 2). They are flask-shaped organs, also protected by an external wall ; each contains a row of three away (fig. 2, B).
The central feature in syngamy is the coalescence of the male and female nuclei; the resulting fusion-nucleus is then diploid, with the 25 number of chromosomes (see CYTOLOGY). That character is then maintained throughout the tissues of the sporo phyte. On the other hand, when the fern-plant comes to maturity and forms sporangia, the cells that are to form the spores undergo a nuclear change, called reduction or meiosis, by which they resume that simpler constitution possessed by the nuclei of the prothallus, and described as haploid (x). These events normally alternate in regular succession, and they constitute that nuclear cycle which underlies all normal life-histories of the Archegonia tae. They stamp structurally the distinction of the two alternating generations. Such alternation, in one form or another, appears in all plants that show sexuality.