116 THE BIOLOGICAL PROBLEM OF TO-DAY 



part played by a cell, as Vochting puts it, will 

 depend upon the position it comes to assume in the 

 whole living unit. To use an expression of Driesch's, 

 dissimilar differentiation of cells is a ' function of 

 position.' Such a conception my brother and I, ia 

 our Studies on the Germ-layer Theory, sought to 

 establish clearly by many examples from the 

 histology of the coelenterates and of higher animals ; 

 such a conception for long has been clearly ex- 

 pressed in physiological botany. 



The simpler nature of plants in structure and 

 function makes it easy to conduct experimental 

 observations upon this point. 



I have already described how either side of the 

 prothallus of a fern may be made to produce male 

 or female organs, according as it is kept in the 

 light or in the dark. Similarly, taking a willow 

 slip, roots may be made to appear at one end by 

 moisture and darkness, while they will not appear 

 on the end kept in the light. 



The experiments of botanists and of fruit-growers 

 show that young buds and the rudiments of roots 

 are indifferent structures, the further growth of 

 which depends entirely upon the conditions in 

 which they are placed. ' One and the same bud 

 may grow to a long or short vegetative shoot, to a 

 floral shoot, to a thorn, or may remain undeveloped. 

 The same root rudiment may grow to a main tap- 

 root or may form a secondary lateral root. The 

 conditions that determine the mode in which these 

 structures will develop are quite within the power 

 of the experimenter. We have shown already 



