LEAVES 



33 



in some deciduous leaves, as those of oaks, there is no 

 special preparation for falling, in most of them a special 

 plate of cells is formed at or near the juncture of the leaf 

 with the stem, known as the cutting-off layer, which gradu- 

 ally loosens the leaf from the stem, so that it falls by its 

 own weight or is wrenched off by the wind (Fig. 29). 



In connection with the deciduous habit there often 

 appears the autumn coloration of leaves, so striking a feat- 

 ure of temperate forests. 

 The colors that appear 

 are shades of yellow and 

 red, either pure or vari- 

 ously intermixed. They 

 are the result of the wan- 

 ing activity of the leaf, 

 the yellow 'mostly being 

 the color of the dying 

 chloroplast, and the red 

 coming from the pres- 

 ence of a new substance 

 manufactured in the en- 

 feebled cells. The pop- 

 ular belief that these 

 colors are caused by 

 frost is only partly true, 

 for they often appear 

 before 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 COn- FlG . 30. The needle-leaves of a pine. 



spicuous conditions. 



19. Leaves of evergreens. In contrast with the decidu- 

 ous shrubs and trees are the so-called evergreens, in which 



