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STOKEY— ANATOMY OF ISOETES $29 



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cells from the vascular axis, in the region in which the cambium has 

 not vet become defined. As the meristematic region becomes localized, 

 forming the cambium, the parenchyma with which the leaf trace 

 phloem is connected is pushed out farther and farther from the 

 vascular axis by the secondary tissues. The phloem can retain its 

 ontinuity with' this region in three ways: the xylem at the base oi 

 ihe leaf may elongate sufficiently to compensate for the secondary 

 growth in that region; there may be a splitting-apart of the tissues 

 of the bundle, permitting the phloem to slide along the xylem; 

 the phloem of the leaf trace will be torn apart, leaving one end con- 

 nected with the "prismatic layer" while the other is carried out into the 

 cortex. Undoubtedly, while the leaf trace is young, there is an adjust- 

 ment by the first method. It should be noted, however, that the leaf 

 traces which are connected with living leaves are comparatively few in 



mber and are found in that part of the stem in which secondary 

 growth is scarcely observable. At some time in the development of 

 each leaf trace there comes a time when it is no longer capable of 

 "tension and is unable to keep pace with the development ot the 

 •tern. At this point the tissues of the leaf trace give way, and the 

 outer part is carried out into the cortex and finally sloughed off, while 

 the base becomes more or less crushed and remains as a dead stump, 

 which in time may be completely buried in the secondary wood. 



While the conditions which have led to the imperfect development 

 °f the vascular tissues cannot be known with certainty, among them 

 are doubtUs- the aquatic habit and the reduction or shortening of the 



tcm - The anatomv of l.oetes does not seem to indicate, as SMirH 

 sa ^ (P. 324), "that the genus Isoetcs represents a more primimc 

 to* of sporophyte than any other vascular plant," but it supports me 



] ew of Scott that "the group has clearly undergone reduction from 



,m e more complex type, and probably from some highly organi/ec 



for m ~r 1 ■ - - ■ - tVl the marked 



bet 



opod, as indicated by the secondary growth, the ma 



er( *Pory, and the somewhat complex organization of the leaves 

 a *» the root-bearing portion of the axis." As a consequence o a 

 shortening or anv reduction in stem development, there would natu- 

 f y * a reduction in the xylem. This might be either in the amount 

 0t thc xylem, even to the extent of the entire disappearance of the 

 aawiary wood, or there might be a reduction in the development 



