THE OUTER TISSUES OF PLANTS. 271 



of the Begonia-leaf, 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. 



