254 PHYSIOLOGICAL DEVELOPMENT. 



as the differences in the conditions vary in degree. Still greater 

 becomes the force of the evidence on finding that these 

 strongly-contrasted parts may, when placed in one another's 

 conditions, and kept in them from generation to generation, 

 permanently assume one another's functions, and, in a great 

 degree, one another's structures. Even more conclusive 

 yet is the argument rendered, by the discovery that, where 

 these substitutions of function and structure take place, the 

 superinduced modifications differ in different circumstances ; 

 just as the original modifications do. The fact that a flattened 

 stem simulating a vertically- growing leaf has its two surfaces 

 alike, while when it simulates a horizontally- growing leaf its 

 upper and under surfaces differ, is a fact which, standing 

 alone, might prove little, but proves much when joined with 

 all the other evidence. And its profound meaning becomes 

 the more obvious on discovering that the same thing happens 

 with petioles when they usurp leaf-functions. 



Finally, when we remember how rapidly analogous modi- 

 fications of function and structure arise in the superficial 

 tissues of individual plants, the general inference can scarcely 

 be resisted. "When we meet with so striking a case as that 

 of the Begonia-\ea,f, a fragment of which stuck in the ground 

 produces roots from its under surface and leaves from its 

 upper surface — when we see that though, in this case, the 

 typical structure of the plant presently begins to control the 

 organizing process, yet the initial differentiations are set up 

 by the differential actions of the environment ; the presump- 

 tion becomes extremely strong that the heterogeneities of 

 surface which we have considered, result, as alleged, directly 

 or indirectly from heterogeneities in the incident forces. 



