No. 464.] STUDY OF THE SALICACEE. 531 
temperature which is so reduced as to permit of but slight ab- 
sorbent activity in the roots; and so it is found that the pecul- 
iar form of the conductive tissue is really developed in correlation 
with the environment of the plant. But it is to be observed that 
the protoxylem is called upon, in the course of its development, 
= to meet another essential factor in the life of the plant; namely, 
its ability to withstand external stresses of various kinds and 
through this the establishment of an upright position for the 
organism. It is quite conceivable that this requirement might 
be met by the evolution of a specialized form of element from 
the meristem without reference to the prior development of 
vessels, and this might be looked for in the angiosperms, but 
such has certainly not been the case in the gymnosperms. The 
somewhat slight modification, and in some senses the reduction 
which the vessels of the protoxylem have undergone in their 
adaptation to the movement of the transpiration current, has 
also permitted of their immediate adaptation to the purposes of 
mechanical support. They have therefore become more fibrous 
in character, while at the same time their walls have become 
sensibly thickened and in corresponding degrees more fully 
fitted to withstand stress. It is thus evident that two forms or 
directions of development have been combined in the same ele- 
ment, and it is no doubt also true that the peculiar form of the 
pits is in itself a direct resultant of such a combination develop- 
ment, since it has been shown that the bordered pit represents 
a localized area which has been left over in the otherwise gen- 
eral thickening of the wall in response to the requirements of 
intercellular circulation. The reduction of the bordered pit and 
its frequent elimination from the wall of the tracheid, especially 
in the summer wood, is in direct harmony with these statements 
and represents the relative predominance of requirements relat- 
ing to circulation or mechanical support at different periods of 
growth and under special conditions of environment. So far as 
we know at present, no further modification of the tracheid 
than that presented in existing species is possible to the gymno- 
sperms, and this particular line of evolution may therefore be 
regarded as completed in that group. 
Our hypothesis as applied to the angiosperms on analogous 
