14 THE SYNTHESIS OF CARBOHYDRATES 



cessation of the process at about noon on a bright day not- 

 withstanding the fact that the conditions for photosynthesis 

 apparently are most favourable. 



The importance of the conditioning factors is obvious, 

 and it is clear that the significance of any one cannot be 

 evaluated without reference to others. 



EXTERNAL FACTORS. 



THE RAW MATERIALS. 



Water and carbon dioxide arc the requisite raw materials 

 for carbon assimilation. 



Water. — Water is essential not only as such for the fabri- 

 cation of food, but also to keep the leaf tissues in a condition 

 mechanically fit for the processes to take place. With regard 

 to the water supply, the transpiration current is the immediate 

 source ; it is, however, not convenient on the present oc- 

 casion to consider the problems presented by this phenomenon. 



Thoday found that the rate of carbon assimilation lessened 

 as the leaves of Helianthus annuus lost their turgidity ; in 

 an extreme instance, when the leaves were very flaccid, the 

 stomates were all but closed and the increase in dry weight 

 was very small indeed. Some determinations by Thoday of 

 the average increase in dry weight of leaves in different con- 

 ditions of turgidity have been mentioned. If cells become 

 plasmolysed, constructive activities must cease ; if in such 

 cells the turgid condition be not recovered, death supervenes. 



Iljin * has obtained results similar to those of Thoday, 

 and his work stresses the indirect action of water, for the loss 

 of water from the leaf results in the closure of the stomates, 

 wherefore the supply of carbon dioxide is diminished and thus 

 the rate of carbon assimilation is depressed. If the loss of 

 water is temporary and not too great, the guard cells regain 

 their turgidity and the stomates open ; but if the loss of water 

 has been excessive, recovery may not take place and the guard 

 cells die, wherefore the efficiency of the leaf suffers. In such 

 conditions, Iljin considers that the injury to the guard cells 



* Iljin : '■ Jahrb. wiss. Bot.," 1922, 61, 670 ; " Flora," 1923, 16, 360. 



