LEAVES 



215 



are the result of the waning activity of the leaf, the yellow 

 mostly being the color of the dying chloroplasts, and the red 

 coming from the presence of a new substance produced in the 

 enfeebled cells. The popular belief that these colors are 

 caused by frost is only partly true, for they often appear 

 before there has been any frost ; but they may be induced by 

 any conditions that tend to diminish the activity of the leaf, 

 and cold is one of the conspicuous conditions. 



In contrast with the deciduous shrubs and trees are the 

 so-called " evergreens," 

 in which there is no regu- 

 lar annual fall of leaves. 

 Such leaves endure for a 

 varying length of 'time, 

 but as there is no regular 

 period for all of them, the 

 shrub or tree always ap- 

 pears in foliage. In the 

 temperate regions, the 

 most conspicuous ever- 

 greens are the pines and 

 their allies. A compari- 

 son between the needle- 

 leaf of a pine (Fig. 181) 

 deciduous tree will show 



FIG. 182. Cross-section of a pine needle, show- 

 ing epidermis with sunken stomata, groups 

 of heavy-walled cells beneath epidermis, the 

 mesophyll (the cells characterized by curious 

 infolded walls) containing seven resin canals, 

 and the central vascular region containing 

 two bundles (xylem uppermost). 



and 

 what 



the leaf of an ordinary 

 the evergreen habit in- 

 volves in temperate regions. The pine leaf must be 

 protected so as to endure the winter, and this has in- 

 volved reduction in surface and extremely thick protec- 

 tive layers about the mesophyll (Fig. 182). This ha& 

 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. In other words, the cost of the winter pro- 

 tection of leaves is a loss of working power during the spring 

 and summer. The deciduous leaf, on the other hand, is 

 broad and thin, with great capacity for work ; but this for- 



