THE RAW MATERIALS 17 



EXTERNAL FACTORS. 

 THE RAW MATERIALS. 



Water and carbon dioxide are the requisite raw materials 

 for carbon assimilation. 



WATER. Water is essential not only as such for the fabrica- 

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

 mechanically fit for the processes to take place. 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 conditions 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. With regard to the water 

 supply, the transpiration current is the immediate source ; it 

 is, however, not convenient on the present occasion to con- 

 sider the problems presented by this phenomenon. 



CARBON DIOXIDE. Under normal conditions, the carbon 

 dioxide for carbon assimilation is derived from the atmosphere 

 and to a lesser extent from the products of respiration.* The 

 amount of respiratory carbon dioxide is conditioned mainly by 

 the temperature and may be equal to half the possible inflow 

 from the atmosphere at the higher temperatures possible in 

 laboratory experiments. The entry of atmospheric carbon 

 dioxide into the plant is either through the intact epidermis, 

 as in those plants which like certain aquatics lack stomates, or 

 mainly through the stomates and, to a much lesser and negli- 

 gible extent, provided the amount of carbon dioxide is not 

 unduly increased, through the unbroken epidermis. This cuti- 

 cular path of gaseous interchange once was thought to be the 



* Under abnormal conditions it appears that plants can make use of carbon 

 dioxide from the soil. Pollacci ("Atti. Inst. Bot. Univ. Pavia," 1917, 17,3) 

 found that plants grown in soil rich in humus or in water culture enriched with 

 carbon dioxide could form starch and increase in dry weight notwithstanding the 

 fact that their aerial parts were in an atmosphere freed from carbon dioxide. 

 The assimilation, however, was not sufficient for normal growth. 



VOL. II. 2 



