542 LIFE-HISTORIES 



reproduction is sometimes accomplished in ferns by the formation 

 on various parts of buds which fall to the ground and take root 

 (see Fig. 368). 



Filicina; in general agree with the ferns described in being arche- 

 goniate, vascular plants, forming true roots and stems, and having 

 alternate leaves upon which are borne sporangia that discharge their 

 spores without elaters. The number of species is reckoned at about 

 3,000. 



193. The scouring-rushes (Class Equisetinae) are repre- 

 sented in modern times only by comparatively small plants 

 of the genus Equisetum (Fig. 369) — about 25 species — 

 which, however, are closely related to numerous gigantic 

 rush-like coal plants, typified by the genus Calamites 

 (Fig. 277, 2). 



In Equisetum cross-fertilization is accomplished by having male 

 and female gametophytes which, as shown in Figs. 370, 371, differ 

 considerably from one another, the female being much the larger 

 and suggesting somewhat by its pseudo-leaves the nurse-plant of a 

 moss. The sporophyte differs remarkably from that of any fern in 

 the comparatively great development of the stem. This is hollow 

 except at the nodes, and performs nearlj^ all the work of photosj'n- 

 thesis. 



The roots do not differ essentially from those of ferns, but 

 the foliage leaves are reduced to toothed sheaths serving 

 chiefly to protect the tender regions of the stem. The fibro- 

 vascular bundles of the stem are arranged in a ring, and in 

 some forms (mostly extinct) a cambium like that of higher 

 plants is develoi)ed which gives rise to successive rings of 

 tissue. Such additional material by which increase in thick- 

 ness is accomplished takes the name of secondary tissue, to 

 distinguish it from the primary tissue formed by the primary 

 meristem. The epidermis is often so filled \vith silica or flint, 

 as to render the plants useful for scouring metal, and this 

 accounts for the popular name. Certain subterranean 

 branches of the rhizoma (a, Fig. 369) may have their funda- 

 mental tissue gorged with reserve food, and thus form tubers 

 which feed new growth in spring, and may sometimes serve 

 as a means of vegetative reproduction. Among the vertical 

 branches there is often a differentiation into the purely 

 vegetative and the purely reproductive. The latter terminate 



