METABOLISM IN LIVING PLANTS. 463 



or by the acids which are present in such abundance, into sugar and various other 

 harmless materials, and the pericarp, which, until now had been sharp, acid, and 

 unwholesome, becomes sweet and luscious. The same coat which formerly served 

 as a protection, now forms an attraction. The ripe fruits, with the seeds they 

 inclose, are now sought for and eaten as food, especially by birds; the sweet 

 covering is digested, while the seeds, excellently protected against the action of 

 digestive juices, are excreted with the waste materials of the food, and germinate 

 in the places where they are deposited; thus the widest dissemination of the 

 plants is brought about. All this will be discussed in detail later when distributing 

 agents of plants are being considered; but it seems appropriate to mention these 

 processes here in order to point out that the metabolism of materials in plants keeps 

 pace with the requirements for the time being; that even when the division of 

 labour in the plants is as much developed as in the cases just mentioned, the 

 arrangements and displacements of the atoms, and the decomposition and formation 

 of chemical compounds, are always carried on in the right place and at the right 

 time, i.e. always where and when the plant is benefited thereby; and that frequently 

 the reasons for all these changes only become intelligible when we consider the 

 inter-relations of animals and plants. 



The significance of salts of sulphuric and nitric acids, as well as the relations 

 of these to oxalic acid, have already been discussed and explained — how by 

 means of the latter the sulphuric and nitric acids are liberated, yielding sulphur 

 and nitrogen for the construction of albuminous substances. If oxalic acid ac- 

 cordingly does not appear to be a necessary plastic constituent of the framework 

 of the plant, it is nevertheless quite indispensable as an accessory to metabolism. 

 The same thing applies to the other organic acids which exist in plants. They 

 are only accessories in the transformations, or intermediate steps between the 

 final stages of the compounds formed in the plants, viz. between the first carbo- 

 hydrate on the one hand, and the completed substance used for building or further 

 purposes on the other. Under these conditions, it is intelligible that the organic 

 acids should be distributed through all parts of the plant, and that the juices 

 in living plants almost universally have an acid reaction. It is also intelligible 

 that the number of organic salts should be extremely large. Oxalic, tartaric, 

 •citric, and malic acids may be cited as examples, but more than two hundred 

 such acids are known which have been detected in various plants. Many of 

 them are also found in animal bodies, viz. isolated members of the series of the 

 so-called fatty acids, which form fats when combined with glycerine, as, for 

 ■example, butyric and formic acids, the latter, as already stated, being also contained 

 in the stinging hairs of nettles. It has, moreover, been already pointed out that 

 glucosides are decomposed by organic acids, and give rise to various kinds of 

 sugar. It is interesting with respect to these sugars that they arise as the first 

 organic products (which result from the assimilation of carbonic acid), and also 

 again as the terminal members of a very long chain of transformations and 

 decompositions of glucosides effected by the action of organic acids. An important 



