412 Prof. H. Marshall Wa,rd. The Relations between 



spoken of two sets of bodies among the many which are produced 

 daring the metabolic processes of the cell — the organic acids and the 

 amide-snbstances. It appears from the evidence to hand that organic 

 acids are not only formed, but are also subsequently oxidised, in the 

 cell, and it is only to be expected that this process of decomposition 

 of the acids is also promoted by raising the temperature, and con- 

 versely,* and such is the case ; these acids increase during the night 

 and diminish during the day, and one important factor in the pro- 

 cesses is temperature. The optimum of increment of organic acids 

 in the plant occurs at somewhat low temperaturesf — e.g.. about 10° C. 

 to 15° C. ; while the minimum coincides approximately with that of 

 respiration (near 0° C.), and the acids cease to increase — or, rather, 

 they are decomposed as fast as, or faster than, they are produced — as 

 the temperature rises to 35 — 40°. 



With respect to the effects of light on metabolism, reference may 

 be made as to what has already been said as regards its promoting 

 certain processes of oxida/fciou in the cells, % and to what follows on 

 assimilation. The part played, by oxygen also has been adverted to ; 

 metabolism in the ordinary course of events depends on respiration, 

 and all that affects the latter affects it. In the absence of free oxygen, 

 conditions of intense destructive metabolism are eventually set up, 

 the details of which we need not discuss. § If plenty of non-nitro- 

 genous food materials are present, the metabolism goes on for some 

 hours as usual, but soon the starving protoplasm undergoes more and 

 more profound changes, resulting eventually in a loss of proteid 

 substances. 



It is important to bear in mind that in the cells containing chloro- 

 phyll the free acids diminish in daylight, and increase as the light 

 fades and in darkness, no doubt because there is less oxygen in the 

 absence of that set free by the chlorophyll corpuscles ; these acids also 

 decrease in proportion as the temperature rises, and increase as it 

 falls. It is also important to be clear in this connexion as to the fact 

 that two processes are going on simultaneously — on the one hand, 

 organic acids are being formed as products of incomplete oxidation in 

 the respiratory processes, and, on the other, they are being further 

 oxidised and decomposed when the temperature is high and the light 

 bright. || Whether at any given moment the amount of acid present 



* See Warburg, ' Unters. aus d. Bot. Inst, zu Tubingen,' vol. 2, Heft 1, 1886, 

 p. 102. 



f Warburg, op. cit. } pp. 71 and 102, confirming the results obtained by de Vries 

 (literature quoted) . 



X See Pfeffer, ' Ueb. die Oxydationsvorgange,' &c, pp. 419 and 454. 



§ See, however, Palladin, " Ueber Eiweisszersetzung in den Pflanzenbei Abwesen- 

 lieit von freiem Sauers^off " ('Ber. d. Deut. Bot. G-esellsch.,' 1888, p. 205). 



j| That the connexion with light depends on the access of oxygen set free in 



