THE FOOD-STUFFS OF PLANTS. 23 



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 now 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. 

 It is not in its elementary form, however, that the nitrogen which we 

 thus find stored reaches the soil, but in all probability as an ammo- 

 niacal compound. This the bacteria then transform into nitrates, 

 which the plant requires for its existence. I do not wish to convey 

 the impression, however, that all plants require their nitrogen in this 

 form, for we know that the Saccharomyces cerevisiaB, for example, 

 can elaborate its nitrogen not only from ammoniacal salts directly, 

 but is even incapable of utilizing that which is furnished in the 

 form of nitrates. Under certain conditions, moreover, probably all 

 plants can, for a while at least, grow in the presence of ammoniacal 

 nitrogen only. 



From what has been said, it is clear that a certain symbiosis exists 

 between the bacteria of the soil and plants, in virtue of which the 

 former transform the ammoniacal nitrogen that is present in the 

 soil into nitrates, which can be utilized by plants, Avhile they in turn 

 probably aid the nutrition of the bacteria by furnishing them with 

 humus and the ternary matter which is necessary for their develop- 

 ment. 



The necessary mineral salts the plant likewise obtains from the 

 soil. 



The question now arises : In what manner do plants effect the 

 synthesis of those complex chemical substances which go to form 

 their tissues from the simple bodies which serve as their food-stuffs ? 

 The kinetic energy which is necessary to effect these changes is, as 

 has been stated, derived from the sunlight and transformed into 

 potential energy by the chlorophyl. We should thus expect to find 

 in those parts in which this is present the origin of those final 

 products which we meet with in the tissues of the plant. These 

 products may be divided into three groups, and in the following 

 pages an attempt will be made to describe the manner in which 

 representatives of each are formed. I shall accordingly consider 

 the origin of the carbohydrates, the fats, albumins, and certain non- 



