THE HIGHER CRYPTOGAMFA. 359 



the tip of the older of the roots which are still hidden 

 within the bark : the middle portion of the root is bent 

 away from the plane of the section (PL LI, fig. 1). 



As in the first year so also in the following ones the bark 

 is pierced by the new roots close under the deepest part of 

 the indentation. The brown roots of the previous year 

 stand far outside those of the current year. Inasmuch as 

 many of them last for two, three, or four years before they 

 become quite decayed, the result is that in some plants, 

 especially the older ones, the peculiarity observed by Von 

 Mohl makes its appearance with the greatest distinctness. 

 This peculiarity is, that the oldest roots are the outermost 

 and apparently the highest, the youngest the innermost and 

 apparently the lowest. As appears from what has been 

 said above, this is only an apparent irregularity, depending 

 upon the unusually vigorous development of the bark, and 

 its yearly renovation from within outwards. 



It is a rule without exception that the middle ones of 

 each series of generations of roots are the oldest, that those 

 which are nearer to the lateral terminal points of the furrow 

 of the stem break forth at a later period than those in its 

 middle point. This circumstance however is not unfre- 

 quently less striking, owing to the fact that the duration of 

 each root is far less strictly limited to any definite period 

 than that of the leaves. The outermost roots of the pre- 

 ceding series are almost always in a state of vitality when 

 the first innermost ones of the next series begin to appear. 

 Old vigorous individuals which form a large number (as 

 many as twenty) of leaves in the course of one year, develope 

 during this period tiuo complete series of generations of 

 roots ; the whole cycle, commencing with the lowest inner- 

 most roots and progressing to the outermost, is formed 

 twice in succession (PL LI, fig. I). In the Isoetes from 

 South Europe and North Africa, which produce an abund- 

 ance of roots, as many as six generations of roots are pro- 

 duced in the same vegetative period. With the close of 

 each cycle of roots a double pair is added to the number of 

 the roots of the previous year, which double pair originates 

 at the horns of the half-moon-shaped lower portion of the 

 mass of wood (PL LI, fig. I). 



