ORGANIC EVOLUTION 



137 



bearing parenchyma is located in the furrows of the 

 branches. The furrows extend up and down, and the 

 chlorophyll bearing cells communicate with the air by pores 

 or stomates arranged in two rows in each furrow. The sup- 

 porting tissues, (aside from the 

 unusualy stiff epidermis), are 

 located in the ridges, which 

 they chiefly compose. The 

 branches are hollow, and the 

 vascular bundles are arranged 

 radially around the main cen- 

 tral cavity, opposite the ridges 

 upon the surface. Each 

 branch is divided into seg- 

 ments by a scries of nodes (at 

 which it breaks when pulled, 

 whence the popular name, 

 "joint-grass"). The arrange- 

 ment of ridges and valleys, 

 of vascular bundles and cavi- 

 ties, of green respiratory par- 

 enchyma in the valleys and 

 the broad bands of support- 

 ing tissue in the ridges, are 

 readily seen in cross sections 

 of fresh stems. The epidermis 

 is covered with secreted nod- 

 ules of silica which render the 

 branches rough to the touch 

 (whence the popular name 

 "scouring-rush") . 



The spores are developed 

 in numerous sporangia that 

 grow underneath the scales of a fruiting cone (fig. 86 a, b, c) 



Fig. 86. The common equisetum {E. 

 arvense). a, a fruiting spray; b 

 and c, two scales from the fruiting 

 cone, showing marginal sporangia; 

 d, two spores, with their hydro- 

 scopic elaters partly unrolled; e, a 

 bit of underground stem, bearing a 

 round tuber. 



