PROTECTION 



245 



nection with this subject. You are familiar with trees 

 like the pine, whose needle-like leaves stay on the year 

 round, and you may be wondering how they avoid injury 

 in winter. One glance at a pine needle does much to 

 answer this question. Both by shape and structure it is 

 evidently far better protected than wide, thin leaves are 

 protected. It is in no such danger from loss of water or 

 from injury by the freezing of the water in it. Instead of 

 being a disadvantage, leaves of this kind are a positive 

 advantage to plants in winter. On many days it is warm 

 enough and bright enough for photosynthesis, and on such 

 days the needle-like leaves are at work. (It is to be noted 

 that the roots of the plants which bear 

 such leaves usually penetrate below the 

 frozen zone of the soil.) In summer, 

 however, such leaves are a disadvantage 

 as compared with broad, thin leaves, for 

 the latter have much greater exposure 

 to light and much greater chlorophyll 

 content. 



Most plants which lose their leaves FIG. 8 5 . Diagram ii- 

 in the fall, as the time to shed them 

 approaches, form a special layer of cells 

 at the base of the petiole. By means of 

 this layer the leaves are shed by a sort 

 of self-amputation. This special layer 

 is called the absciss layer. (The word 

 absciss means cutting-off.) It loosens the 

 leaf from the stem, and forms a place at 

 which any gentle breeze or even its own 

 weight is likely to detach the leaf. (See Figure 85.) After 

 the leaf falls, the wound which might be expected is often 



lustrating prepara- 

 tion for leaf fall. A 

 section through the 

 base of the petiole 

 of a mature leaf ; a 

 indicates the absciss 

 layer. This layer 

 runs through the 

 cortex, indicated by 

 c. The leaf is held 

 till the last by its 

 vascular strands, v. 



