62 AIR A^•D LIGHT. 



cal alterations in the sap caused by exposure to 

 light. 



The same thing occurs in regard to nutritive 

 matter, which in like manner is formed by the 

 exposure of leaves to light. Thus the Potato 

 when forced in dark houses contains no more 

 farinaceous matter than previously existed in 

 the original tuber ; but acquires it in abundance 

 when placed in the light, and deposits it in pro- 

 portion as it is influenced by light and air. 

 Thus, also, if Peaches are grown in wooden 

 houses, at a distance from the light, they will 

 form so little nuti'itive matter as to be unable 

 to support a crop of fruit, the greater part of 

 which will fall off. And for a similar reason 

 it is only the outside shoots of standard fruit 

 trees that bear fruit. Considerations of this 

 kind form in part the basis of pruning and 

 training. 



Light is the most powerful stimulus tliat can 

 be employed to excite the vital actions of plants, 

 and its energy is in proportion to its intensity ; 

 so that the direct rays of the sun will produce 

 much more powerful effects than the diffused 

 light of day. 



Hence, if buds, that are very excitable are 

 placed in the shade, their excitability will be 

 checked. 



