1 84 



BOTANY OF THE LIVING PLANT 



but at points on the shoot, which are usually determined by the 

 external conditions. The roots sometimes lap round the support, 

 with a prehensile action, as in many of the large Aroids. 



The familiar case of the Virginia Creeper (Ampelopsis) is morpho- 

 logically identical with its relative the Grape-Vine, but the tendrils 

 are attached by adhesive discs. The tips of the branched tendril 

 move away from the light, and this leads to 

 contact with the support : a rock, wall, or 

 tree-trunk. After contact each tip widens 

 into a disc, which at first secretes an ad- 

 hesive cement. This together with its very 

 close application to the inequalities of sur- 

 face gives a firm attachment. Subsequently 

 the tissues harden, and the tendril may 

 assume spiral curves, which give a spring- 

 like resistance (Fig. 137). 



Correlation of Growth. 



The examples of external adaptation thus 

 selected show that the Vegetative System 

 of the Higher Plants is liable to various 

 modifications of form and appearance, and 

 that these often have definite relation to 

 the surroundings under which the plant 

 grows. But such modifications are subject 

 to the limiting principle of Correlation. 

 Correlation of Growth involves the fact 

 that where one part is developed larger 

 than usual another part is liable to be 

 This applies especially to the shoot, and it 

 may be illustrated by many familiar examples. The succulent stem 

 of a Cactus (Fig. 132, p. 176), distended for water storage, bears cor- 

 relatively small leaves. The same is seen in the swollen tuber of the 

 Potato, with its correlatively reduced scale-leaves (Fig. 138.). An 

 extreme and peculiar case is that of Welwit^chia, where two enormous 

 plumular leaves increase in size through a long term of years, but the 

 main axis which produced them is atrophied. 



Correlation applies not only between leaf and axis, but also between 

 the various parts of the leaf. Thus in the young Broad Bean, and still 

 more clearly in the genus Baiihinia, the two basal pinnae develop 



Fig. 137. 

 Climbing shoot of Ampelopsis 

 Veitchii. The tendrils {R) have 

 attached their adhesive discs to the 

 wall-surface behind them. (After 

 Strasburger.) 



correspondingly reduced. 



