MANUAL OF NATURE 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. In 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. They breathe thesame as we do with 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. 
