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, 



,-a 



FIG. 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. 



