32 OUR NATIVE FERNS AND THEIR ALLIES. 



87. The furrowed stems are hollow, except in E. scirpoides, 

 and in addition to the large central cavity there is a series of 

 smaller air-cavities opposite the furrows known as the vallecular 

 canals, the furrows themselves being called valleculcE and the 

 ridges carince. Opposite the carinae there are still smaller cavi- 

 ties known as carinal canals. The carinae vary in number from 



Figs. 21, 22. — Equisetum silvaticum L., showing sterile and fertile stems. 

 (From Thome.) 



five to fifty in different species. The stems are also jointed, and 

 at each node some species produce a whorl of branches which 

 may be simple or compound. Some species, however, like the 

 common scouring-rush {E. hiemale), produce simple stems. 



88. The leaves are produced also at the nodes, and by the 

 union of their margins form a short sheath which ends in a row 



