OP PLANTS AND ANIMALS. 159 



artificial heat, yet it is impossible to .replace the splendor 

 of the southern sun. 



In tropical countries, the rays of the sun fall perpendi- 

 cularly, and therefore his light is much more intense 

 there than in the temperate or polar regions. As we pass 

 from the equator to the poles, the incidence of the rays 

 becomes more oblique, and consequently, their brightness 

 and stimulating power on the vegetable and animal creation 

 must be diminished in the same ratio. 



All the effects of light on vegetation are not yet fully 

 understood. In temperate climates, in early spring, the 

 temperature depends in a great measure on the prevailing 

 currents of air. If these currents come to us from the 

 north, although the sky is cloudless and the vernal sun 

 smiles cheerfully once more on the leafless forests and the 

 flowerless fields, yet the cold will prevent the development 

 of vegetable life. But are the plants wholly uninfluenced 

 by the light in such circumstances ? It seems possible 

 that, independently of the heat, the constantly increasing 

 light may have a somewhat stimulating effect on vegeta- 

 tion. We are not aware of any facts which confirm this 

 suggestion, but the subject is deserving of attention. 



It is well known that plants grown in a window turn 

 their leaves to the light, and that the pots in which they 

 are kept require to be turned occasionally in order to pre- 

 vent them from growing all on one side. The daisy and 

 dandelion are supported on a long stalk, if their flowers 

 grow in rank grass ; and become sessile on a shaven lawn, 

 or in spots to which the light has free access. Light also 

 influences the position of leaves on the stem, which are 

 always arranged so as to be most favorably situated for its 

 reception. The opposite leaves of labiate plants which 

 usually cross each other at right-angles, develop in the 



