22 INTRODUCTION. 



is absorbed by the remaining bands, V, VI, and VII, of which V is 

 seen to the right of F, VI most marked about G, while VII occu- 

 pies the extreme violet end. Very concentrated solutions allow the 

 red rays to pass only as far as B, while in greater dilution the green 

 rays likewise appear. Such solutions, therefore, appear green when 

 viewed with transmitted light, while with reflected light they are 

 red and fluorescent, 



When a fresh leaf is similarly examined, a spectrum is obtained 

 which is essentially the same as that just described. There is lack- 

 ing, however, the band that corresponds to the red fluorescent rays 

 of chlorophyl solutions. This is explained by the assumption that 

 the red rays* are absorbed by living chlorophyl and transformed into 

 chemical energy. In accordance with this view, we find that when 

 living plants are successively exposed to the various rays constitut- 

 ing sunlight, decomposition of carbon dioxide with liberation of 

 oxygen which, as we shall presently see, takes place in the green 

 portions of every plant whenever it is exposed to sunlight occurs 

 with special intensity when the plant is exposed to the rays corre- 

 sponding to the bands I, II, and III. In this manner, then, chloro- 

 phyl-bearing plants derive their kinetic energy from sunlight, and 

 thus become enabled to elaborate the simple food-stuffs which are 

 at their disposal into the complex substances which constitute their 

 tissues. 



The Food-Stuffs of Plants. The most essential elements which 

 enter into the composition of the tissues of plants are, as has been 

 pointed out, carbon, hydrogen, oxygen, and nitrogen. These sub- 

 stances are available to the plant as carbon dioxide, water, and cer- 

 tain nitrates. The origin of the first mentioned is, of course, obvious, 

 while that of the last is at first sight somewhat obscure. 



The nitrates are present in any soil which contains organic 

 matter, and are known to result from this through the special 

 activity of certain bacteria. Decomposing animal and vegetable 

 matter is, however, not the only source of the nitrates, for it can be 

 demonstrated that arable soil, apparently devoid of vegetable life, is 

 capable, unless sterilized, of fixing a very considerable amount of 

 nitrogen, which must of necessity be derived from the atmosphere. 

 The assumption has been that this nitrogen is obtained as an ammo- 

 nium compound, which the bacteria then transform into nitrates. 

 In this connection it is interesting to note that the hydroxides of 

 some of the heavy metals (iron, cobalt, nickel) are capable of pro- 

 ducing nitrites in minute quantities from the nitrogen of the atmos- 

 phere, and that the first step in the formation of nitrates may hence 

 be a purely chemical one. We do not wish to convey the impression, 

 however, that all plants require their nitrogen in this form, for we 

 know that Saccharomyces cerevisise, for example, can elaborate its 

 nitrogen from ammonium salts directly, and is even incapable of 

 utilizing that which is furnished in the form of nitrates. Under cer- 

 tain conditions, moreover, probably all plants can, for a time at 

 least, grow in the presence of ammonium nitrogen only. 



