The Utilization of Foods 87 



Conditions for growth. The conditions most favorable for 

 growth are abundant water supply and warm temperatures, 



FIG. 51. Results of an experiment to show effects of light and moisture on the growth 

 of potato shoots : A , light but no water ; B, light and water ; C, water but no light. 



such as normally occur in summer. For the growth of the 

 plant as a whole, strong light is favorable because it increases 

 the supply of food. For the growth of leaves in particular, 

 medium light is generally most favorable. In darkness the 

 blades of many plants do not expand, and in very intense 

 light they do not expand fully because of excessive water 

 loss. 



The growing regions of leaves. By watching the develop- 

 ment of leaves on any common herb, or on the trees in spring, 

 we can see that growth takes place rapidly ; also, that growth 

 ceases when the leaves have developed to a certain rather 

 definite size. After the leaf is mature, further enlargement 

 will not take place, no matter how favorable to growth the 

 external conditions may be. The question arises, do all parts 

 of the leaf enlarge equally, or do some parts grow more than 

 others ? There is one characteristic of growing tissue that will 



