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 carina. Opposite the carinae there are still smaller cavi- 

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



FIGS. 21, 22. Equisetum sylvaticum 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 



