21 i 



UUTANY OF THE LIVING PLANT 



Correlation of Growth. 



The examples of external adaptation thus selected show that the 

 Vegetative System of the Higher Plants is liable to various modifica- 

 tions 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 



Fig. 147. 



Lower parts of a Potato plant, Solatium tuberosum. The swollen tuberous stems 

 bear correlatively small scale-leaves. (After Baillon, from Strasburger.) 



larger than usual another part is liable to be correspondingly reduced. 

 This applies especially to the shoot, and it may be illustrated by many 

 familiar examples. The succulent stem of a Cactus (Fig. 141, p. 209), 

 distended for water storage, bears correlatively small leaves. The 

 same is seen in the swollen tuber of the Potato, with its correlatively 

 reduced scale-leaves (Fig. 147). An extreme and peculiar case is that 

 of Wehvitschia, where two enormous plumular leaves increase in size 

 through a long term of years, but the main axis which produced them 

 is an enlarging stump, bearing no further leaves upon it. 



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

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

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

 to a large size, while the distal part of the leaf is represented only by 



