Ways in Which Plants Increase in Size 349 



earlier described in this book), that I give to it here so much atten- 

 tion. There is, however, another reason for its consideration, 

 namely, that Darwin considered it the starting point for most of 

 the useful plant movements, the twining, sleep, geotropic, hydro- 

 tropic and other adjustive movements which we considered under 

 Irritability. His conclusion on this point, has not, however, been 

 accepted by later investigators, though the present status of the 

 matter may be expressed by saying that his view is unproven 

 rather than disproven. 



Finally, as to the physiological phases of growth, there remains 

 one matter which is both scientifically interesting and econom- 

 ically important, and it concerns grafting. Everybody knows 

 that small twigs of apples, cherries, pears and many other plants 

 can be cut from those trees and inserted into the stems of others 

 in such way as to grow and form structurally an integral part of 

 the new tree. Furthermore (and this is what gives to grafting its 

 great economic importance), the inserted twig and everything 

 which subsequently grows from it, continues to produce its own 

 kind of leaves, flowers, and fruits substantially unaffected by the 

 plant into which it was grafted; while, correlatively, the stock 

 plant into which the graft was inserted continues to produce its 

 own kind of vegetation unaffected by the graft, even though this 

 may in time become the greater part of the tree. Thus it is 

 possible to graft a number of very different varieties of apples, or 

 of cherries, into a single trunk and produce a tree which bears 

 all those varieties as long as it lives, without any visible sign to 

 show that it was ever anything other than one tree from the start. 

 It is in this way that highly specialized forms of fruits, leaves, or 

 flowers, which appear mainly as sports (to be further considered 

 in our chapter on Plant Breeding), and which cannot be grown 

 from seed, are propagated and multiplied indefinitely. 



Turning now to the purely physiological side of grafting, the 

 first fact of prominence is this, that the twig, which is called the 

 scion, (or don), and the plant into which it is inserted, called the 



