CONSTRUCTION OF THE SHOOT SYSTEM 53 



leaves and the more delicate parts of the twig. These 

 openings, which are known as stomata, are themselves 

 co-ordinated with the regulation of this exhalation of 

 vapour, the width of the opening being capable of 

 variation according to different conditions. We must 

 associate the evaporation of so much vapour by the 

 leaves with the very large absorption of water we 

 observed in the root, and we can see that the structure 

 of the leaf is as well adapted to evaporation as that of 

 the root is to absorption. Further structural adapta- 

 tions to this maintenance of a stream of water through 

 the plant will become evident later, but in the mean- 

 time we can see in the features already alluded to a 

 definite relation to this particular interchange between 

 the plant and its surroundings or environment. 



Still pursuing our inquiry, we may notice that while 

 the general colour of the shoot is green, the depth of the 

 green tint is not uniform. The flattened parts or leaves 

 are of a brighter green than the cylindrical axes, and in 

 general it soon appears that the more exposed any part 

 of the shoot is in its young and most delicate condition, 

 the more prominent the green colour becomes. We 

 have consequently a suggestion of some co-ordination 

 between exposure and colour. Comparing two shoots 

 growing in different places we can soon associate the 

 optimum brightness with the best illumination, and we 

 are led to infer that one reason for the flatness of certain 

 parts of the shoot is the desirability of exposing as much 

 of their surface as possible to light. 



We have already called attention to the fact that the 

 greater part of the plant's food is manufactured in the 

 leaves, and that the green colouring matter chlorophyll 

 is chiefly concerned in making it. The chlorophyll is 

 not diffused throughout the living substance, but is 

 confined to a number of small ovoid bodies which are 

 embedded in it, and these green bodies are placed very 



