CH. IV.] THE LEAVES. 151 



stautly keep in mind that, with eveiy leaf that he 

 removes, he deprives the plant of a primary organ of 

 its existence. 



Light, it has just been stated, is the cause of the 

 green colour of plants ; but it should be obseiTed that 

 its full power is only beneficial when directed upon 

 their upper surface. This is evidenced by the posi- 

 tion they always maintain. Trees nailed either to a 

 north or south wall, or trained as espaliers, always turn 

 the upper surfaces of their leaves outwards, to where 

 there is most light. Plants in a hot-house uninflu- 

 enced by the direction from whence proceeds the first 

 supply of air or the greatest degree of heat, turn not 

 only their leaves but their very branches towards 

 the source of brightest light, and if not turned 

 almost daily, entirely lose their sjTnmetrical form. 



If the branches of a tree trained against a wall, or 

 other support, are so moved when their leaves are 

 completely expanded, that the under side of the 

 foliage is the most exposed to the light, they are 

 always found to regain their natural position in a 

 day or two. If the experiment is often repeated on 

 the same individual, the leaves to the last continue 

 to revert, but become gradually weaker in the effort, 

 partially decay, and their epidermis peels off. Suc- 

 culent leaves are particularly sensible of light, but 

 those of pinnated, legTiminous plants, as the pea and 

 French bean, are still more so. 



