CONSTRUCTION OF THE SHOOT SYSTEM 55 



that this evaporation discharges. In the time of sun-, 

 shine a great deal of the sun's energy in the form of heat 

 and light is falling on the plant. It has been computed 

 that the amount is so great that it would raise its 

 temperature to such a dangerous extent that if no 

 counter-influence were at work it must speedily perish. 

 Now the evaporation of water always requires the 

 expenditure of a considerable amount of heat, and we 

 find that the greater part of the heat reaching the 

 plant from the sun is devoted to the vaporisation of 

 the water in the intercellular passages of the leaves and 

 other parts. The normal temperature of the plant is 

 thus maintained in the face of the enormous absorption 

 of solar heat which its exposed and often unprotected 

 position renders inevitable. 



The study of the behaviour of the chloroplasts shows 

 us that their position is definitely associated with the 

 duty which we have attributed to them. Not only is 

 their colour dependent on their exposure to light, but 

 the part they play in the construction of food is equally 

 related to the illumination they receive. We have 

 already spoken of the work done by the chloroplasts, 

 and have seen that they construct organic food in the 

 form of sugar and .similar compounds from the carbon 

 dioxide of the air, together with a portion of the water 

 supplied them by the root. Carbon dioxide is present 

 in very small proportion in the air, only some 3 or 4 

 parts in 10,000. The construction of food from such 

 antecedents is only possible in the presence of light; 

 two things therefore must be secured a wide surface 

 and preferably a copiously subdivided one, to bring as 

 much air as possible into contact with it, and as com- 

 plete an exposure as possible of the chloroplasts to 

 light to enable the construction of the sugar to go on. 



The form and disposition of the shoot system must 

 be regarded from the point of view of these require- 



