150 



especially characteristic. The structure of it is not markedly 

 xerophytic; the inner layers are collenchymatously thickened as 

 is the case with the sieve-tissue cork, in the secondary cortex, 

 in the older stem. 



The secondary cortex shows only a slight development of 

 cork-tissue; the outermost layers become corky, but they cohere 

 but imperfectly. The slight development, in the whole of the 

 cortex of xerophytic tissue, is a natural result of the oecological 

 form of the plant, which causes the stems not to come in 



direct contact with the atmosphere, 

 and the leaves to be closely set 

 and to fall off late -features which 

 make all specially xerophytic tis- 

 sues in the stem unnecessary. 



In transverse section the ele- 

 ments of the wood, with the ex- 

 ception of the vessels and some 

 cells of the medullary rays, are 

 peculiarly folded^ as if the stem had 

 experienced pressure from several 

 directions (Fig. 7). The reason for 

 this Grevel states is quite unknown 

 to him, as it also is to me. It 

 is possible that this compression 

 Fig. 6. Diapensia lapponica. '^ °0^ ^^ be seen in the living 



Transverse section of stem; slightly mag. plant. 1п loUgiludiual SeCtiOU it 



(Julianehaab.) 



can be seen that the cells do not 

 lie exactly in the direction of the longitudinal axis of the stem, 

 but are placed obliquely, which gives a very confused appear- 

 ance to the section. Probably this is in a measure connected 

 with the close-set leaves, the frequent formation of adventitious 



1 This shows most distinctly in preparations mounted in common glycerin, 

 but not so well in Canada balsam preparations, such as those from 

 which the photographs were taken. 



