I02 VASCULAR CRYPTOGAMS 



tendency dis])layed by the epidermal cells to deposit silica in their cell- 

 walls. It is always divided into ver}' distinct nodes and internodes, and 

 is furnished at the nodes with modified foliar organs of a membranous 

 character, the leaf-sheaths^ the margin of which is split into a number of 

 teeth. The tissue of the internodes is permeated by large intercellular air- 

 canals. At the nodes the stem (except in some species the fertile stem) 

 gives out a whorl of symmetrically arranged branches, which almost 

 precisely resemble the main stem except in their smaller size and simpler 

 structure, consisting of internodes and nodes furnished with leaf-sheaths ; 

 but these secondary branches do not usually again branch. The stem and 

 root increase in length by means of a single large pyramidal apical cell, 

 which produces three rows of segments. The ' vascular ' bundles of the 

 stem are but feebly developed, and contain but little xylem. Both stem 

 and branches perform the function of leaves, contain chlorophyll, and 

 are provided with stomates. The sporanges are never borne on the 

 branches or leaves, but are collected into spicate or catkin-like ' fructifi- 

 cations,' borne at the extremity either of ordinar}^ vegetative stems, or of 

 special fertile stems which resemble the ordinary stems in structure, but 

 contain no chlorophyll and do not branch. The sporanges spring from 

 the inner side of the peltate scales of which these ' fructifications ' are 

 composed : they dehisce by a longitudinal fissure, but have no annulus 

 like those of ferns. The spores are distinguished from those of all other 

 Vascular Cryptogams by being enclosed in four distinct coats, the outer- 

 most of which ultimately breaks up into four ribbon-shaped strips, and 

 detaches itself from the spore except at the point of junction of these 

 strips, which are termed elaters. The elaters are remarkably hygroscopic, 

 absorbing or giving off water with every change in the moisture of the sur- 

 rounding atmosphere. In consequence of this they are constantly altering 

 their shape, and imparting a somewhat rapid motion to the spores, thus 

 assisting in their dissemination. On germinating the spore gives rise 

 to a strap-shaped prothallium, which has an independent power of 

 growth, and is usually dioecious. The antherids and archegones differ in 

 no essential point from those of other Vascular Cryptogams. Male and 

 female prothallia are generally produced in close proximity to one 

 another, so that impregnation is readily effected through the agency of 

 moisture. 



The oophyte generation of Equisetum (L.) springs directly from the 

 spore, which contains chlorophyll. On germinating the spore throws off 

 its outer coats, and changes its form from spherical to pear-shaped. The 

 contents, still clothed in the thin endospore, then divide by a wall, the 

 direction of which is not constant, into two cells of unequal size ; accord- 

 ing to Stahl the direction of this division depends on the direction of the 

 rays of light. From the smaller of these two cells the chlorophyll 



