LEAVES AND LEAF STRUCTURE 



89 



and, in the case of the broad-leaved plants, this shedding also comes 

 because it is necessary to reduce the exposed surface so that too 

 much water may not be evaporated in the winter, when the roots 

 can supply but little. Of course one can see another reason for 

 plants that grow in climates where snow prevails during the winter. 

 The weight of snow accumulated by the leaves would tend to 



break the plant down. 



In the case of the pines 

 with their slender 

 needles this reason does 

 not apply. 



The color changes of 

 autumn are not due to 

 frost entirely, but may 

 be caused by anything 

 which stops the activity 

 of the plant. The 

 beautiful yellows and 

 reds that make our 

 autumn a blaze of glory 

 act as a protection to the 

 sensitive green sub- 

 stance of the leaves, 

 which is being withdrawn and stored for use another year. 



Before the leaves of a plant fall there is formed at each leaf 

 base a waterproof layer (abscission layer) which prevents the loss 

 of sap after the leaf is gone. 



The enormous amount of ashes left when the leaves are burned 

 gives some idea of the amount of unused mineral matter which 

 the plant had stored there, and incidentally reminds us that plant 

 ashes, whether from stems or leaves, are useful food materials for 

 plants and ought to be put back on the soil for use another year. 



MINUTE STRUCTURE OF LEAVES 



The chief function of the leaf is the manufacture of food ma- 

 terials. To understand this, a thorough study of the minute 

 structure is necessary. 



FIG. 23. The same plant at sundown 

 showing the head turned to the west. From 

 Atkinson. 



