[Chap. XXI INTERRELATIONS OF THE PARTS OF A PLANT 189 



are fewer than 50 leaves per apple. For the largest Elberta peaches 

 about 30 leaves per peach are necessary. For the best cluster of grapes 

 there should be not less than 12 to 15 leaves per cluster. 



The roots and underground stems of troublesome weeds, such as 

 dandelion and thistle, will die of starvation if the green tops are removed 

 frequently and immediately after they begin to develop, or if they are 

 densely shaded by another plant such as alfalfa. In interpreting this 

 fact it should be remembered that the synthesis of starch, fats, and 

 proteins may occur in roots and underground stems, and that any of 

 these three substances may be transformed to sugar in the cells of these 

 organs. An adequate interpretation of this fact and of several of those 

 that follow involves, therefore, an explanation of why the roots and 

 underground stems cannot make starch, fats, or proteins and survive 

 upon them as food when the tops of the plants have been removed or 

 when the rate of photosynthesis is continuously low. 



A good growth of Irish potato tubers is not obtained if the potato 

 plants are continuously exposed to high temperatures ( Fig. 66 ) or dense 



Fig. 66. Effects of different temperatures on the formation of potato tubers. Photo 



by John Bushnell. 



shade, or if the soil contains an abundance of nitrates. Under any of 

 these conditions the tops (aerial stems and leaves) may grow fairly 

 well; with a high nitrate supply there is usually a luxuriant growth of 

 tops. Under experimental conditions a low yield of tubers also results 



