In some of the fern-allies like Marsilia and Selaginella the plants produce two kinds of spores, (1) microspores, which develop into prothalli producing only antherids, and (2) macrospores, which develop into prothalli pro ducing only archegones. In these plants, par ticularly Selaginella, the prothallus is much re duced in size, and the sexual reproduction ap proaches that of the pines and other conifers, except that it takes place away from the plant producing the spores. The microspores of Selaginella are thus the exact counterparts of the pollen-grains of the pine, and the macro spores correspond to what is known as the embryo-sac in the young pine-cone, to that part of the ovule in which the egg-apparatus ap pears, and in which the seed is ultimately developed.
Ferns are distributed all over the world but diminish in the number of species, in the pro fusion of individuals, and in the relative pro portion to the total vegetation of the locality as we pass from tropical to polar regions. Moist insular climates are best adapted to their devel opment. Java has 575 species of ferns, while New York State with about the same area has scarcely one-tenth as many. Jamaica, with about the area of Connecticut, has nearly 500 species; Hawaii, Samoa, New Zealand and the Philippines are other prolific regions. While moist conditions favor fern-growth and devel opment, certain species have adapted themselves to dry climates and grow on open exposed rocks of semi-arid regions. Such ferns are provided with woolly hairs, tangled scales, waxy powder or other mechanical contrivances to conserve their moisture. One species of the so-called "resurrection plants" (Selaginella lepidophylla,' of the arid regions of the Southwest conserves its moisture during the dry season by coiling its branches inward so as to form a compact ball. On the return of moisture it flattens out and exposes its fresh green surface. The curls up in a similar way, its under surface being protected by a layer of scales. It grows on trees in the Southern States and the West Indies.
The ferns of temperate regions are terres trial in their habit, but as we approach the tropics a large number become epiphytic, grow ing with mosses and orchids on the trunks and branches of trees. The filmy ferns frequently cover the trunks of tree-ferns completely.
There are over 5,000 described species of pteridophytes in the present flora of the world, which are grouped according to their charac ters under seven distinct orders. Of these we have nearly 290 species within the limits of the United States.
Geologically, the pteridophytes are a very old group dating back to the Devonian and reaching their first culmination during the Carboniferous Age, in which they formed the bulk of the vegetation of the coal-measures. Many of the fossil formerly described as ferns, however, are really primitive seed-plants closely allied to the ferns. (See PTERTDOSPERMS). Be sides ferns, some of the fern-allies of those times attained very great size, as shown in the giant trunks of Lepidodendron and Sigillaria, re lated to the modern club-mosses, and in the large stems of Calamites, related to our present di minutive scouring-rushes. The various types of the modern pteridophytes and their distinctive characteristics will be treated under their re spective orders, arranged, as nearly as a lineal series can show it, in their relative rank in the scale of vegetable life.
I. Filicinm. A. Eusporangiata.— The adder tongues and grape-ferns are eusporangiate; that is, their sporangia develop from the interior tissues. They include plants of a fleshy tex ture, bearing their thick-walled sporangia in spikes or panicles. The prothallus, so far as known, is subterranean and colorless. We have in the United States seven species of adder tongue (Ophioglossum), and 17 species of grape-ferns (Botrychium), the former having the sporangia in spikes and the latter having them in panicles. All our northern species are terrestrial plants with fleshy roots. Cheiro glossa palmatia of Florida and the West Indies is a hanging epiphyte growing on palmetto trunks and on other trees, and the curious strap-like Ophioderma of Hawaii and other Pacific isles grows in similar situations, with its pendant leaf from two to five feet long. The order OphioglossoJes represents the sim plest type of fern-growth in existence, and probably the oldest type as well. The species (about 60) are not numerous, but are very widely distributed.
The Marattiaceze include only tropical ferns of a coarse habit, ranging from 2 feet to 10 feet in height. They are eusporangiate like the last order, but have the coiled-bud form of the true ferns. They also bear their sporangia dorsally, that is, on the under surface of the leaf, but these are formed in special boat-like receptacles known as synangia. Marattia and Dancra are the two principal genera. Two spe cies of the former and nine of the latter are found in the North American tropics. The order is a smaller one than the last and, like it, consists of a single family.