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 



