Chap. V] 



THE SOIL 



in 



There can be no doubt that the fungus derives certain nutritive materials from its host. 

 That, conversely, matter passes from the fungus into the cells of the host, is proved at the 

 death of the vesicles, which shrivel up as they give out a liquid. It was not possible to 

 determine what the latter contains in solution, or what is the composition of the 

 granular mass that remains in the dead vesicle and is not utilized by the host. 



Most plants provided with a mycorhiza obtain from it, in any case, only 

 a portion of the carbon that they need. Some plants however, especially 

 those growing in deep forest shade, are entirely dependent on the mycorhiza 

 and have lost their chlorophyll. They, like fungi that nourish themselves 

 directly on humus, are termed saprophytes. Plants that contain chlorophyll 

 but nevertheless require the organic constituents 

 of humus are IiemisapropJiytes, an intermediate 

 stage between true saprophytes {Jiolo saprophytes) 

 and completely autotrophic plants. Sapro- 

 phytes will be discussed in a future chapter. 



iii. CHEMICAL DIFFERENCES IN HUMUS 

 AND THE RESULTING FLORA. 



The floras of mild and of acid humus are 

 quite dissimilar. Many species may be at once 

 described as characteristic of the one or the 

 other kind of humus ; for instance, for mild 

 humus, Asperula odorata, Mercurialis perennis, 

 Milium effusum, Melica uniflora, Stellaria 

 nemorum ; for acid humus, Aira flexuosa, 

 Maianthemum bifolium, Melampyrum pratense, 

 and several mosses, such as Hylocomium tri- 

 quetrum, Polytrichum formosum, Leucobryum. 

 On the very acid humus of moors the vegetation 

 assumes a decidedly xerophilous character, because 

 the humous acids impede the absorption of zvater 

 by the roots. 



Mild and acid humus are collective terms for numerous kinds of humus 

 that vary according to the nature of the decomposing plants. The 

 differences between them are more easily discovered by the fine chemical 

 analysis of plants than by the rough chemical analysis of our laboratories. 

 Each kind of humus has its characteristic species of plants. There are 

 plants depending on the different kinds of humus, as on the mineral 

 constituents of soils — some confined to one kind of humus, others that are 

 indifferent. Many species of plants grow only on the humus of coniferous 

 forests ; for instance, Goodyera repens and the North American saprophyte 

 Schweinitzia odorata. Monotropa Hypopitys occurs in broad-leaved 

 forests almost exclusively in its glabrous form, in coniferous forests in its 





r> 



Fig. 60. Thismia Aseroe. 

 Two cells of the mycorhiza. 

 After P. Groom. 



