354 THE FOOD OF PLANTS 



for nitrate- and nitrite-bacteria are able to assimilate carbonic acid chemo- 

 synthetically and perhaps by an entirely different form of organic synthesis. 

 It is possible, by chemical means, to produce carbohydrates from carbon 

 dioxide by various processes of chemical synthesis, so that the photosyn- 

 thesis of the latter need not always occur in the same manner. It is indeed 

 always a fundamental error to suppose that a living organism with definite 

 specific properties will necessarily produce a given substance in the way 

 which seems probable to us from our present knowledge of chemistry and 

 physics (Sect. i). 



The almost instantaneous recommencement of the evolution of oxygen 

 in light, and the rapidity with which starch may reappear, do not neces- 

 sarily indicate that the number of intermediate processes must be extremely 

 limited ; for various machines are known which form elaborated products in 

 a very short space of time although a large number of actions intervene 

 between commencement and completion. The end-product affords no 

 evidence as to whether the preceding operations have all taken place be- 

 tween the plasmatic micellae or molecules of the chloroplastid, or whether at 

 certain stages plasmatic compounds have been formed, whose disintegration 

 has resulted in the production of sugar or other substances. However 

 this may be, the chloroplast is able by utilizing the energy of sunlight 

 to produce organic substance from carbon dioxide and water without 

 any perceptible diminution of its own bulk. It is the task of physiology 

 to explain as far as possible the means by which this is accomplished, and 

 what is the precise function of the different parts, including chlorophyll. 

 At present, however, it is not even known whether chlorophyll simply acts 

 as a sensitizing' agent, and by transferring the intercepted light-vibrations 

 to colourless parts of the chloroplastid renders these capable of assimilating 

 carbonic acid, or whether it is of use in other ways and perhaps takes a 

 direct part in the decomposition of this gas (cf. Sect. 60). 



The evolution of free oxygen and the formation of less highly oxidized 

 substances from carbon dioxide and water may be distinct, though closely 

 connected, processes, even if the intermediate changes be very limited 

 in number. Neither physiology nor chemistry can afford any conclusive 

 evidence as to what is the first act in the process. Light may indeed cause 

 oxygen to be evolved from oxide of mercury, but it may also cause reduction, 

 as, for example, in hydroquinone, and it is, moreover, capable of inducing 

 various decompositions, or even polymerization in certain substances 1 . 

 Hence it is possible that, in the different stages of assimilation, light may 

 at one time exercise a reducing, at another an oxidizing action. 



The entire process is certainly not directly connected with the presence 



1 See Klinger, Ber. d. Chem. Ges., 1886, p. 486; Ostwald, Lehrb. d. allgem. Chem., 1893, 

 2. Aufl., Bd. i, p. 1085. 



