110 PLANT LIFE 
on the lines just indicated have other functions 
besides purely mechanical ones, and these 
must be adequately discharged if the plant 
is to be a success. As soon as the foliage 
is produced, water. is imperatively demanded, 
and thus there arises, so to speak, a conflict 
between opposing requirements. The scleren- 
chymatous tissue is excellent for supplying 
the needed strength, and its axile position 
renders it very effective. But it is of little 
or no use as water-conducting tissue. Now 
as a matter of fact we find that in the higher 
types of climbers (e. g. many members of the 
natural orders Leguminose, Sapindacee, 
Bignoniacee, etc.), that this strong flexible 
axile core is succeeded externally, and quite 
suddenly, by vessels of wide calibre which, 
though admirable as water conduits, are 
practically useless regarded from the stand- 
point of material strength. But the latter 
defect loses all significance as the plant grows 
older, for the difficulties that were to the fore 
in the climber’s earlier life become obviated 
later on in a very simple manner. If one 
observes an old climber in the jungle, the 
lower part of the stem is often seen to be 
lying in snaky coils on the ground, and is 
evidently not at all exposed to any serious 
tractive forces. The peculiarity in question 
is due to the fact that up above, in the roof 
of the forest, the lower leafy branches of 
the climber are dying back as they give place 
to the younger ones springing nearer the 
