CLIMBING AND WATER PLANTS 117 
for the individual species. The most special- 
ised of these organs are the tendrils. 
Tendrils may be formed from. specially 
modified branches as in the passion flower, 
or from leaf-stalks as in clematis, or from the 
leaves or leaflets as in various species of 
vetch, and even from roots as in the vanilla 
orchid. But they all tend to become very 
similar in form, and to assume in common just 
those characters that enable them so well 
to discharge their functions. 
But the very fact of their diverse origin 
(from leaves, stems, etc.) in the different 
plants suffices to emphasise the importance 
of what we may call the internal living 
factor, as opposed to the environment directly, 
in their production. And this is further 
strengthened by the circumstance that they 
are produced fully formed, they are not 
gradually and _ tentatively produced and 
perfected during their development, any more 
than are any of the historically older organs 
of the plant. But nevertheless, many of 
them are endowed with the faculty of further 
growth in thickness and strength if they 
become functionally active. This power is 
not restricted to tendrils but is of widespread 
occurrence, and is especially obvious in the 
case of the stalks of heavy fruits. These, 
like functional tendrils, greatly increase the 
amount of mechanical tissue primarily present 
in their tissues as the fruits increase in weight. 
The advantage secured is in both examples 
