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- Fic. 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 
