MANUAL OP NATURB STUDY. 85 



of equal size, age and strength; from the one group 

 cut off all the blades from the petioles, repeating the 

 operation as often as new blades appear. Iri every 

 other respect the same favorable conditions should 

 surround both groups of plants. At the end of one 

 week note the difference in growth of main stalk 

 both in thickness and in length. Why do potato 

 beetles injure the growth of the potato? What 

 effect do caterpillars have upon the growth of plums, 

 apples, peaches, gooseberries? Why? The leaf, 

 then, must be a sort of factory auxiliary to the seed 

 factory. It is in the leaf that all the material 

 brought up by the roots or absorbed through the 

 stomata is made over into plant substance. But 

 our experiments prove that this factory cannot work 

 in the absence of sunlight and proper temperature 

 and moisture. Sunlight, then, is one of the factors in 

 this factory. But these leaves do more than take in 

 water. Theybreathethesameas we dowith our lungs. 



Leaves also serve as shade for the protection of 

 the delicate young shoots or branches against the 

 scorching rays of the sun. Also to give slope to 

 carry the rainfall and dew to proper places, where 

 the plant can be served to best advantage. In fact 

 the whole upper surface is more or less concave, 

 somewhat like the palm of our hand, and therefore 

 capable of holding water. 



