LIGHT AND HEAT 73 
The Colour of Young Leaves and Shoots.—In some 
cases young leaves are, for some time after emerging from 
the bud, reddish in colour. As the leaves grow older, 
this pigmentation gradually passes away, and the natural 
green colour is assumed. The pigment is contained in 
the sap, and is protective. The phenomenon is chiefly 
observed among plants in the Tropics, where the light is 
very strong. Certain rays of the spectrum—chiefly the 
blue and the actinic, which are very abundant in strong 
light—act injuriously upon the chlorophyll, especially in 
young leaves. The red sap, by absorbing these rays, 
Fic. 26.—TRANSVERSE SECTION OF SHADE-LEAF OF WHORTLEBERRY. 
a, cuticle ; b, epidermis; c, chlorophyll-tissue ; d, air-space ; e, stoma. 
screens the chlorophyll from damage, at the same time 
allowing the red rays, to which it is transparent, to pass 
into the assimilating cells. 
Heat and Cold. 
But for the heating effect of solar radiation, the world 
would be so cold that life of any kind would be impos- 
sible on its surface. In the neighbourhood of freezing- 
point most of the vital activities manifested by plants 
sink to zero. A further increase in cold becomes danger- 
ous, and in most cases a point is soon reached when the 
aerial organs are killed. Some plants, however, can 
withstand a great deal of cold. In one case, recorded 
from Siberia, a plant was suddenly caught in the grip of 
winter while in full flower; the sap froze hard, and 
animation was suspended for many months. When the 
frost broke, the plant thawed and renewed its activity at 
the point where it left off, just as if nothing had occurred. 
