Gaura 

 parviflora 



Sunshine 



Solidago 

 (goldenrod) 



Mountain 



Atitr Uiuc'iil/er^. Ihv blori/ ol Evolution Aftci Clements 



INFLUENCE OF ENVIRONMENT UPON PLANTS 



Effects of excessive sunshine are shown in the first pair of plants. Effects of low tem- 

 perature and excessive loss of water are shown in the second pair. 



We have come to take chemical influences for granted in all proto- 

 plasmic activities, both as foods and as poisons. We have also come to think 

 of the vitamins and hormones as chemical modifiers of protoplasm. But 

 growth is not the same as development, and the two processes are not 

 necessarily influenced in the same way by any particular chemical. 



Inner Factors Temperature, light, moisture, chemicals, oxygen, and 

 the like influence metabolism in many species. But what is it that brings 

 about differentiation in the first place ? One way of thinking about what 

 happens during the progressive change from a single cell (or a few similar 

 cells) to the many millions of differentiated cells is to follow cell-divisions 



step by step. 



When two daughter cells are formed, they are apparently just alike. But 

 if they remain clinging together, each has a surface flattened against the 

 other. These cells are no longer round, the same in all directions. After a 

 second division, the four cells press against one another at different relative 

 parts. After a division takes place in a horizontal plane, the food supply 

 is different for the upper cells from what it is for the lower ones. Each 

 cell comes to be influenced in a different way by pressure, food supply, 



358 



