uaA STOKEY-AN ATOMY OF ISOETES 3*7 



fast to show signs of crushing, but those in the upper part are large 

 and later form canals by the loss of the thickened portions. In the 

 upper part of the leaf the protoxylem is usually limited to a single 

 large vessel, whose place can be recognized in old leaves by the large 

 canal formed by its disappearance {fig. 13) 5 but in the lower part of 

 the leaf, as well as in the leaf trace, the protoxylem cells are smaller, 

 trend in number, and less sharply marked off from the met ^ x >, le ^ 

 Fig. 10 represents the section of a young leaf near the tip, in which the 

 only indication of the protoxylem is the large central cell, which is not 

 vet thickened, while the phloem is represented by several cells. he 

 phloem consists of very long and slender tubes whose sieve plates .are 

 terminal. They come out clearly in sections stained in anihn blue 

 or Bismarck brown. Although several preparations suggest the 

 presence of thin areas on the lateral walls, the evidence is not con- 

 clusive. In the portion of the leaf above the ligule there is a very 

 abrupt decrease in the amount of xvlem, but not a corresponding 

 decrease in the phloem. The phloem is well developed throughout, 

 but instead of forming a band on one side of the bundle, as in e 

 lower part of the leaf, it forms an arc. The phloem does not disappear 

 from the center of the arc, leaving two lateral groups such as KRi C 

 h_ a.^, , . r „ . . j Duriaei) and /. ve lala, but the amount 



variable. Near the top of the leaf the 



"' ine center ot the arc is variable, neai w* «r — 



Phloem is sometimes most abundant in the middle of the arc. 



k- 14 is shown a leaf trace from the cortex at the base of e • 



rv,, . r . «< -if travPfsPS 



Thi 



the 



— « 10 wit ijpc uj. UUIlUie WI11LI1 JS ivunvx in , 



he outer part of the cortex, and in the leaf itself from its base 



:~..i . •'...« - ohmntlv into the 



ligule. J 



reduced type of fig 



,c auced tvpe of fig. i 3 . In the former, tne u^— > 

 toty uniform in size, are scattered among parenchyma ««• 

 ^e abundant protoplasm and large nuclei. Before the ^^ 



■ mature, the outer vessels, the protoxylem, begin to coliaj* * 

 ** ^ of bundle the phloem is always in a band and she*- 

 ^ency to surround the xylem. The amount of phloem is less 

 in ^e upper part of the leaf, and by the time the leaf » matur e 

 *** entirely obliterated bv crushing. In foiling the to 

 ! hr ough the cortex toward the center of the stem the tirst <W_ 



■ * condensation of the bundle by a reduction in the amount ot par . 



