XII EQUISETINEJE 467 



green cells are concave outwardly and lie l)eneath the ridges. 

 In secondary branches the amount of this tissue is much greater 

 and the lacunae less conspicuous, or indeed even wanting. 



The epidermis, as is vvell known, contains great quantities of 

 silica, which gives it its very rough and harsh surface. This 

 is deposited either uniformly, as is usually the case in the lateral 

 cell walls, or in tubercular masses. Upon the inner surface of 

 the guard cells of the stomata it forms regular transverse bars 

 (Fig. 271). Upon the outer walls of the epidermal cells the 

 masses form either isolated bead-like projections or these are 

 more or less completely confluent. 



The stomata are peculiar in structure, and their development 

 v^as first correctly described by Strasburger (i). In E. tcl- 

 mateia these only occur usually upon the foliar sheaths, but in 

 species with green internodes they are found principally upon the 

 sides of the furrows over the green hypodermal tissue.^ Before 

 the stoma proper is formed, the cell divides twice by longitudinal 

 walls (Fig. 271), and the original cell is thus divided into a 

 central one (the real stoma mother cell) and two narrow lateral 

 accessory cells. The central cell now divides again, and the 

 division wall splits in the centre as usual. A cross-section of 

 the young stoma (Fig. 271, D) shows that the walls by w'hich 

 the accessory cells are cut off are inclined, so that the stoma 

 cell is broader at the bottom than at the top, and as develop- 

 ment proceeds the accessory cells completely overarch the stoma, 

 and in the older ones look as if they had arisen by horizontal 

 divisions in the primary guard cells. The accessory cells show 

 the same tuberculate silicious nodules upon their outer walls as 

 the other epidermal cells, and upon the inner face of the real 

 guard cells only are formed the regular bars. Stomata are quite 

 absent from the rhizome, and also from the colourless fertile 

 branches of E. tchnatcia. Compared with the aerial stems, the 

 rhizome shows a smaller number of vascular bundles, and a cor- 

 responding reduction in the number of the lacunae. 



The Branches 



Until the researches of Janczewski (3) and Famintzin (i) 

 it was supposed that the lateral branches arose endogenously. 



^ Miss E. A. Southworth (i) found that in E. arvense they occur upon 

 the ridges, and upon the fertile as well as the sterile shoots. 



