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. 
he 
Zen! 
In transverse section the ele- 
er 
ments of the wood, with the ex- 
» 
© 
es 
(2 
oY 7 
® 
er , 
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. is not to be seen in the living 
Transverse section of stem; slightly mag. plant. In longitudinal section 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. 
