io8 THE FACTORS [Part I 



that rain-water collects on it in puddles. With persistent rain, however, 

 it becomes full of water like a sponge, but without yielding any water to 

 the underlying mineral soil. 



Acid humus arises whenever the supply of oxygen is only slight, namely 

 on soils below stagnant water, but also in dry sunny stations, where earth- 

 worms are rare, for these animals would prevent the humus from caking 

 into a solid mass. For the same reason, peat does not contain the mineral 

 constituents of the subsoil which are intimately mixed with the humus by 

 the burrowing activity of the organisms inhabiting mould. 



Wet peat, or peat in the ordinary sense of the word, is characteristic of 

 moors, dry peat of heaths. The latter may be differentiated from moor 

 peat, as heath peat. Dry peat is also found in forests, as soon as the soil 

 has dried after a clearance of the wood and the worms have died out ; in 

 this way the first step is taken towards the conversion of a forest into 

 a heath. Except for the amount of water they contain, the difference 

 between the heath peat and moor peat does not seem to be very notice- 

 able. The elevated and drier parts of moors bear essentially the same 

 vegetation as true heaths on dry soil. 



ii. THE MYCORHIZA. 



Mould and peat are penetrated in all directions by an extraordinary 

 tangle of mycelial threads belonging to various forms of fungi, hitherto 

 rarely identified and appearing to differ from one another in the different 

 kinds of humus. These fungi can exist not only as parasites but also as 

 saprophytes, and form, as they envelop the roots of the higher plants, the 

 so-called mycorhiza, which appears to possess a high significance in the 

 physiological processes of the nutrition of many forest and heath plants ; 

 it is in fact probable that the fungus acts upon the organic components of 

 the humus and partly transfers them in an assimilable form to the roots. 



Mycorhiza was discovered by Kamienski in Monotropa Hypopitys 

 (Figs. $6, 57) and in Fagus sylvatica (Fig. 58) and its importance recognized. 

 Later, Frank as well as Wahrlich, Johow, Schlicht, Oliver, Groom, Janse, 

 and others demonstrated the constant appearance of mycorhiza on many 

 other phanerogams and on pteridophytes, some of them green and some 

 not green, and it was assumed that these plants required the mycorhiza for 

 their normal existence. The name mycorhiza was invented by Frank. 



The fungus of mycorhiza forms either, as a mere epiphyte, a thick 

 coating round the root, which in such cases is devoid of root-hairs, or it 

 lives within the root as an endophyte. In both cases the hyphae are 

 connected with the mycelium ramifying in the soil and belonging in certain 

 established cases to recognized species of fungi. Wahrlich recognized 

 species of Nectria (N. Vandae and N. Goroschankiniana) in the mycorhiza 

 of several orchids, whilst Noack, Reess, and Fisch recognized in Elaphomyces 



