542 LIFE-HISTORIES 



reproduction is sometimes accomplished in ferns lay tlic formation 

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

 (sec Fig. 36S). 



Filicinse 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 com]5aratively small plants 

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

 which, however, are closely related to numerous gigantic 

 rush-like coal plants, t\'pified by the genus C'alamites 

 (Fig. 277, 2). 



In Equisetum cross-fertilization is accomphshed by having male 

 and female gametophj^tes 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 nearly 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 fil)ro- 

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

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

 plants is developed which gives ri.se to successive rings of 

 tissue. Such additional material by which increase in thick- 

 ness is accomplished takes the name of scccuukirn tissue, to 

 distinguish it from the priniury tissue formed by the primary 

 meristcm. The epidermis is often so filled Avith silica or flint, 

 as to render the plants useful for scom-ing metal, and this 

 accounts for the popular name. Certain subterranean 

 branches of the rhizoma (a, Fig. 3()9) 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 i-eproduction. Among the vertical 

 branches there is often a differentiation into the purely 

 vegetative and the purely reproductive. The latter terminate 



