222 



PTERIDOPHYTA. 



larger and less twisted. On the last curve is situated a more or 

 less elongated appendage of cytoplasm (Fig. 223 c). 



The asexual generation. The embryo is similar to that of 

 the Ferns. The fully developed Equisetum. is a perennial herb, 

 with widely creeping (in some species tuberous) rhizome, from 

 which extend erect, aerial, most frequently annual shoots. 



The vegetative aerial STEMS are divided into a number of inter- 

 nodes by the whorls of leaves (Fig. 224). The internodes are hol- 

 low, the cavities being separated from each other by the transverse 



FIG. 221. Equisetum arvense : a fertile branch with cone ; b vegetative shoot; c cone ; 

 d sporophylls. 



partitions of the solid nodes. The lower portion of the internode, 

 which is encased by the leaves, has much thinner and softer cell- 

 walls, so that the stem is easily separated into segments just above 

 the nodes. Each internode has a large number of ridges and 

 furrows, and bears at its apex a whorl of leaves whose number and 

 position correspond to the ridges of the internode. As in the 

 case of other Verticillate plants, the whorls are placed alternately, 

 one above the other ; the same arrangement is also found in the 

 ridges on two successive internodes. In addition to the large 

 air-cavity in the centre of each internode (the central cavity), a 



