PHENOMENA CONNECTED WITH GROWTH. 23 



Not less important than water is air. Without 

 it the sap would not be converted into cambium. 

 The roots decay when withdrawn from the in- 

 fluence of the air. Soil must, therefore, be per- 

 meable. 



Without light trees could not feed, nor produce 

 fruit. Light accelerates the maturing of the sap, 

 by causing an evaporation of the superabundant 

 water in the cellular tissue of the leaves. The 

 roots are thus stimulated to absorb more liquid 

 nourishment. The more light, the more root 

 action. A bright light is needed for the leaves to 

 decompose the carbonic acid accumulated in their 

 tissue. Lis;ht flavours and colours the fruit. 



Heat is also an indispensable agent in vegeta- 

 tion. It stimulates the energy of plants, especially 

 when combined with moisture. Even during 

 periods of great heat, the sap, then much stimu- 

 lated, brings into the leaves some of the coolness 

 of the soil derived through the roots, thus pre- 

 serving the leaves from being injured by the solar 

 rays. During the winter, on the contrary, the sap 

 circulates very slowly, but still brings with it some- 

 thing of the latent earth heat, which is higher 

 than that of the atmosphere, and thus counter- 

 balance its coldness. 



