32 



OUR NATIVE FERNS AND THEIR ALLIES. 



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

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

 smaller air-cavities opposite the furrows known asxhevallecular 

 canals, the furrows themselves being called vallecula: and the 

 ridges carina'. Opposite the carinse there are still smaller cavi- 

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



Figs. 21, i.7..—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 brandies wliich 

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

 common scoiiring-rush (E. hiemale), produce simple stems. 



8S. The leaves are produced also at the nodes, and by the 

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



