PHYSIOLOGICAL DOMINANCE 105 



partially inhibited gradients (see pp. 178-81). Certain 

 external conditions, such as moisture and darkness, 

 favor the development of roots, but do not determine 

 their origin. It is commonly stated by botanists that 

 roots may arise on any or almost any part of a plant 

 where external conditions permit their development 

 or where the need for them exists. This is >true in a 

 sense, because most plants are composite individuals, 

 and when one of the constituent individuals of the plant, 

 such as a bud, branch, or leaf, is sufficiently isolated 

 from an existing root system, or under certain external 

 conditions, that individual may develop a root or roots. 

 Physiologically or physically isolated parts of a plant may 

 undergo transformation into stem-tips without relation 

 to other parts and the stem-tips determine the formation 

 of other parts, but even though various parts of plants 

 may give rise to roots in the absence of stem-tips, in no 

 case does any other isolated part of a plant undergo 

 transformation into roots alone. Moreover, in develop- 

 ment in nature roots and rhizoids in general arise only 

 after the primary apical region has been determined. 

 They are, in short, subordinate to the individual as a 

 whole, but, like leaves and various other plant "organs," 

 possess a certain degree of individuation of their own. 

 The question of the nature of the correlative influence of 

 the root system upon other parts of the plant is one of 

 considerable interest and is touched upon in chap, v 

 (pp. 159-63). 



THE RECONSTITUTION OF AN INDIVIDUAL FROM AN 

 ISOLATED PIECE 



In the case of Planaria dorotocephala it has been 

 possible to analyze the process of reconstitution to some 



