A TEXT-BOOK OF BOTANY 



there is no regular annual fall of leaves. Such leaves en- 

 dure for a varying length of time; but as there is no regular 

 period for all of them, the shrub or tree always appears in 

 foliage. In the temperate regions the most conspicuous 



evergreens are the pines and 

 their allies. A comparison 

 between the needle-leaf of a 

 pine and the leaf of an ordi- 

 nary deciduous tree will show 

 what the evergreen habit in- 

 volves in temperate regions 

 (Fig. 30). The leaf of a pine 

 must be protected so as to 

 endure the winter, and this 

 has involved reduction of 

 surface and extremely thick 

 protective layers about the 

 mesophyll (Fig. 31). This, 

 has diminished the ability to 

 work; but it has saved the 



tree the necessity of putting out a complete new crop of 

 leaves for the next season. The deciduous leaf, on the 

 other hand, is broad and thin, with great capacity for 

 work; but this forbids protection during the winter. 



20. Special forms of leaves. Besides the ordinary leaves 

 that have been considered, and which are called in distinc- 

 tion foliage leaves, there are special forms of leaves whose 

 chief work is different. In so far as they are green, they 

 manufacture carbohydrates as do the foliage leaves, but a 

 distinct change in structure and behavior indicates that this 

 is not their chief work. 



(1) Scales. The most conspicuous illustrations of 

 leaves that have become modified into scales are to be found 

 in subterranean stems and scaly buds. Underground stems 

 cannot produce foliage leaves on account of the absence of 



m 



FIG. 31. Cross-section of the needle- 

 leaf of a pine, showing the epider- 

 mis with heavy cuticle (e), in which 

 are sunken stomata (s); masses of 

 heavy-walled cells (h) beneath the 

 epidermis; the mesophyll region (m) 

 containing chloroplasts, and the cen- 

 tral colorless region containing two 

 vascular bundles. After SACHS. 



