THEORY OF THE DEVELOPMENT OF ORGANISMS 119 



deprive it of roots, then the same cells that in the 

 other case produced leaves and shoots will now 

 produce roots. Precisely the same occurs with a 

 piece of the root. In fact, it appears as if the idio- 

 plasm knew what parts of the plant were wanting, 

 and what it must do to restore the integrity and 

 vital capacity of the individual.' 'The idioplasm 

 in the remaining part of a plant must be affected 

 when an important part has been removed, because 

 the idioplasm of the lost part is no longer capable 

 of having influence.' * It is clear enough that 

 necessity acts as a stimulus, and that each definite 

 need calls into existence the appropriate reaction.' 



These are Naegeli's views, and they have been 

 elaborated by Pfliiger in his important treatise on 

 The Teleological Mechanism of Living Nature 

 (1877). 



Vochting writes in similar fashion : 

 ' In a tree that is growing under normal condi- 

 tions, without being subjected to injury, all the 

 organs appear in definite relation to each other: 

 so many leaves correspond to a definite number 

 of twigs and branches. These spring from a stem 

 of proportionate thickness, and the stem passes 

 into a definitely proportioned tap-root, from which 

 arise a due array of lateral roots. In normal 

 conditions all these organs are in equilibrium. An 

 apple-tree, growing on the line where tilled garden 

 ground meets a lawn, grows more vigorously on 

 the side towards the garden. If one of the roots of 

 an apple-tree with three main roots and three 

 branches be amputated, then the correspond- 



