44 o THE FOOD OF PLANTS 



water 1 . A similar result would be produced if the addition of chalk 

 neutralized the acidity of the nutrient fluid, and thus induced conditions 

 unfavourable to the development of plants which grow best in acid media 

 (Sects. 73, 74). 



In nature various combinations of external conditions may be presented, 

 and these are continually changing, so that it is difficult or impossible to 

 determine precisely the different factors at work in producing a particular 

 result. A single factor sometimes becomes especially prominent, as, for 

 example, in the case of saline plants, but care must always be taken not to 

 ascribe to a single cause a result which may be due to the combined action 

 of several. General physiology deals only with the fundamental principles 

 by which the organic world is governed, and hence to give an account of 

 the problems of plant distribution is beyond our scope. 



The causes influencing the distribution of heterotrophic plants are even 

 more complicated and involved than those affecting autotrophic plants, to 

 which the discussion has hitherto been confined, for in both antagonistic 

 and reciprocal symbiosis a whole series of complicated relationships are 

 necessarily introduced. Similarly saprophytes are largely restricted to 

 particular nutrient media, according to their requirements and their powers 

 of utilizing different nutrient substances (Sects. 64-67). Moreover the 

 formation and excretion of certain metabolic products may ultimately lead 

 to an accumulation of substances which inhibit further growth, either of the 

 same or of other organisms, while at the same time a suitable medium may 

 be provided for the development of new forms of life. This phenomenon is 

 especially obvious in the case of fungi and bacteria, and a few remarks have 

 already been made concerning it (Sects. 64 and 92). Thus the accumulation 

 of butyric acid, or of alcohol, ultimately inhibits the further fermentative 

 activity of the organisms concerned, while in a solution of sugar either 

 fungi or bacteria may gain the upper hand according to the acidity or 

 alkalinity of the medium. Similarly in the process of putrefaction several 

 different organisms follow one another, and this is not only the case in the 

 rapid decomposition of proteids, but also in the slower disintegration of 

 a dead tree. 



As autotrophic plants gradually construct a fertile soil rich in humus 

 from sterile sand or bare rocks, progressive series of plants follow each 

 other at prolonged intervals (Sects. 28 and 51). With the appearance of 

 forest trees the increasing shade renders impossible the development of light- 

 loving plants beneath them. Again, the gradual exhaustion of a given 

 constituent from the soil may cause the original inhabitants to be replaced 

 by others suited to the changed conditions. It is, however, improbable 



1 Pfeffer, Bryogeograph. Stud. n. d. ihatisch. Alpen, 1869, p. 126 (Sep.-abdr. a. d. Detikschr. d. 

 Schvveizer Naturf.-Ges.). 



