120 THE BIOLOGICAL PROBLEM OF TO-DA Y 



ing branch will lag behind in growth, although it 

 may not absolutely perish.' * The equilibrium 

 varies according to the specific nature of the tree. 

 It is shown in one way in the oak, in another in the 

 beech, and is different in the varieties of a species.' 



Finally, consider this statement from Goebel's 

 Treatise on the Morphology and Physiology of the 

 Leaf: 'The fact that lateral buds do not develop 

 while the axial bud is still growing vigorously 

 depends upon the relation between the two. That 

 I denote as correlation of growth.' 



The dependence of parts upon each other, and 

 upon the whole, is specially clear and instructive in 

 cases where different plant individuals are united by 

 budding or grafting. To limit the growth of a 

 tree, and to induce it to become dwarfed, it is 

 necessary only to graft it upon a nearly allied but 

 dwarf variety. When a pear-tree is grafted upon 

 the quince, which is characterized by its dwarf- like 

 growth, the vegetative growth of the pear is re- 

 duced exceedingly. It produces shorter and 

 weaker shoots ; all the dwarf varieties of the pear 

 employed as wall fruits, or growing into the little 

 pyramids spoken of in the trade as ' cordon '-trees 

 and potting-trees, could not have been produced 

 unless the gardener had had the quince as a 

 natural dwarf stock (Vb'chting). With the dwarf- 

 ing is associated a freer and earlier production of 

 fruit. Other kinds of fruit-trees, apples, apricots, 

 and so forth, show the same course. 



* The capacity to withstand external influences 

 and the duration of life may be altered in the same 



