THE ISOETACE^. 211 



tuber-like, has a central vascular bundle surrounded by a 

 region of growing cells that annually produce new bark 

 on one side and add to the vascular tissue on the other. 

 At two or more points this tissue is more active than at 

 others, with the result that mature rootstocks are either 

 two, three, or morelobed. In the hollows between these 

 lobes, the roots are given off, and these are peculiar for 

 forking two or three times like the selaginellas, instead of 

 branching irregularly as those of most plants do. 



The leaves number from ten to two hundred and are 

 arranged spirally on the upper part of the rootstock. 

 At the point where they join it they are wide 

 and somewhat triangular in cross-section, but 

 above they approach cylindrical in form. In 

 length they range from less than two inches to 

 more than two feet. Running lengthwise through 

 the centre of the leaf is a bundle of bast which, 

 with the surrounding tissue, 

 separates the hollow interior 

 into four parallel chambers 

 containing many cross-parti- 



Lross-section of leaf 



near base. tions. In addition to the cen- 



tral strand of bast there are frequently four 

 others, called peripheral bast bundles, near the 

 surface. One of these is at the back on the 

 outer side, another is in the middle of the inner 

 face, and the two others are found on either side Ieaf 

 of this on the inner edges of the leaf. In most S spoTa" 

 of our species the peripheral bast bundles are glum ' 

 absent ; when two are present they will be found to be 

 the dorsal bundle and the one in the middle of the 

 inner face. The two on the inner edge of the leaf are 

 usually absent. 



