CHAl'. V] 



THE SOIL 



III 



Tlwe 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, ivhich 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 niycorhiza 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 themycorhiza 

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

 directly on liumus, arc termed saprophytes. Plants that contain chlorophyll 

 but nevertheless require the organic constituents 

 of humus are hcmisapropJiytcs. an intermediate 

 stage between true saprophytes [Iwlosapropliytes] 

 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 

 aemorum ; for acid humus, Aira flexuosa, 

 Maianthemum bifolium, Mclampyrum pratense, 

 ind several mosses, such as Hylocomium tri- 

 :iuetrum, Polytrichum formosum, Leucobryum. 

 In the very acid hiniuis of moors the vegetation 

 issiimes a decidedly xerophiloiis eharacter, beeaiise 

 he humous acids impede the absorption of ivater 

 >y the roots. 



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

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

 lifferences between them are more easily discovered by the fine chemical 

 uialysis of plants than by the rough chemical analysis of our laboratories, 

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

 'lants depending on the different kinds of humus, as on the mineral 

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

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

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

 chweinitzia odorata. Monotropa H}'popit\-s occurs in broad-leaved 

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



Fig. 6o. Thismia Aseroe. 

 Two cells ot the niycorhiza. 

 .A.fter P. Groom. 



