20 



SOIL CONDITIONS AND PLANT GROWTH 



the concentration of the cell sap becomes high. Synthesis of protein, 

 on the other hand, is not restricted to the chlorophyll cells nor is it 

 directly dependent on light,^ but it does not start as low down as 

 carbohydrate synthesis, the initial substances being, apparently, sugar 

 and nitrates. 



For the translocation of food materials from one part of the plant 

 to another they have to be changed, if necessary, into simpler soluble 

 substances, starch being converted into sugars and protein into cer- 

 tain decomposition products ; this change is effected by enzymes. For 

 storage purposes it is usually necessary that the substances should be 

 insoluble and they are therefore reconverted into complex bodies. 



The destructive process, respiration, in which oxygen is absorbed, 

 sugar oxidised and carbon dioxide evolved (in rather less volume 

 than that of the oxygen) is a general property of protoplasm. It 

 takes place throughout the whole life of the plant and in all the living 

 cells; during the growing period it is of course on a i smaller scale than 

 the synthetic processes, but during both germination and ripening it 

 is on a larger scale, consequently there is a loss of weight. 



These separate processes — assimilation, translocation, metabolism, 

 respiration — all seem to follow the ordinary laws of chemical reaction, 

 the only modification necessary being the introduction of a time factor, 

 since protoplasm will not indefinitely maintain its powers. For in- 

 stance, in the classical experiments of Blackman and of Miss Matthaei 

 (194), the effect of temperature on assimilation, all other factors being 

 eliminated, was precisely that obtaining in an ordinary chemical reac- 

 tion ; so also for respiration. Miss Matthaei found that the amounts 

 of carbon dioxide assimilated by a cherry laurel leaf per 50 sq. cms. 

 (about 8 sq. ins.) per hour at various temperatures were : — 



By interpolation the values at 0°, 10°, 20° etc. can be found, and the 

 rate of assimilation is thus seen to double, and more than double, for 

 every increase of 16°, the usual order of increase in chemical reac- 

 tions : — ^ 



1 Confirmed by Zaleski's recent experiments (323). 



^ The normal rates were only maintained for a short time at the higher temperatures. 

 'A list of the papers dealing with the temperature coefficient for cell growth is given 

 in Science, 6th November, 1908. 



