46 THE PHYSIOLOGY OF PLANTS 



This fact can be illustrated by experiments in which plants are 

 treated in one case with humus in its raw or fresh condition, 

 and in the other with humus which had been subjected to the 

 action of steam (2 1 2° Far.) for several hours. The same quan- 

 tity of soil after the action of the heat yields a crop many 

 times in excess of the former. 



The mechanical effect of the humus upon the structure of 

 the soil we shall deal with in speaking of stable manures, but 

 to the general consideration of the efficacy of humus we must 

 append an account of the curious modification of the roots of 

 many of the humus-loving plants. This special form of root 

 is termed myeorhiza, and it differs from the normal root 

 structure by the fact that the lateral rootlets, instead of being 

 long and slender, are thick and short, more branched and 

 interlocked, and form a nest-like structure. These rootlets, 

 too, are usually devoid of root-hairs, so essential for absorption ; 

 but their entire surface is covered by a dense felted mass of 

 fungal threads, so that the root itself does not seem to be 

 directly in contact with the soil at all, but can only take up 

 its nutriment by means of the fungal threads. As the root 

 grows in length at its apex, the pseudo-parenchymatous threads 

 of the fungus increase in number too, so that the slightly 

 developed root-cap is constantly covered in by a dense cap of 

 fungal hyphae. This fungus being devoid of chlorophyll cannot, 

 as has been proved, absorb or utilise any raw or inorganic salts, 

 and probably derives all its nourishment from the organic com- 

 pounds of the humus. If, therefore, we do not like to assume 

 that this fungal covering becomes mechanically saturated with 

 the inorganic nutriment of the soil and passes this on to the 

 cells of the root, we will be forced to the conclusion that a large 

 number of cultivated plants actually derive their nourishment 

 from the organic substances of the humus, and do this either 

 by preference or by necessity. 



How widely distributed in nature this arrangement of my- 

 eorhiza is may be gathered from the fact that all our forest 

 trees, such as oaks, beeches, birches, alders, Conifers, and also 

 poplars, willows, and limes, are provided, where they grow in 

 humtis, with inycorhiza. They will, however, develop normal 

 j-ootlets and root-hairs if they are grown in solution or in 



