132 CLIMBING PLANTS. 



off ; but when the discs are formed and attached, 

 the tendril becomes very strong. 



" Plants become climbers, in order, as it may be 

 presumed, to reach the light, and to expose a large 

 surface of their leaves to its action and to that of 

 the free air. This is effected by climbers with 

 wonderfully little expenditure of organized matter, 

 In comparison with trees, which have to support 

 a load of heavy branches by a massive trunk. 

 Hence, no doubt, it arises that there are so many 

 climbing plants in all quarters of the world, be- 

 longing to so many different orders. These plants 

 have been arranged under four classes, disregard- 

 Ing those which merely scramble over bushes with- 

 <out any special aid. Hook-climbers are the least 

 efficient of all, at least in our temperate countries, 

 .and can climb only in the midst of an entangled 

 mass of vegetation. Root-climbers are excellently 

 adapted to ascend naked faces of rock or trunks 

 <of trees; when, however, they climb trunks, they 

 are compelled to keep much in the shade ; they 

 cannot pass from branch to branch and thus cover 

 the whole summit of a tree, for their rootlets 

 require long-continued and close contact with a 

 steady surface in order to adhere. The two classes 

 of climbers and of plants with sensitive organs, 



