VII.] MYCORHIZA 205 



The term " mycorhiza " is applied to the symbiotic 

 combination of the filaments of certain fungi, whose 

 complete development is as yet unknown, with the 

 finest rootlets of certain plants. Sometimes the fungus 

 forms a sort of cap on the exterior of the short root- 

 lets, which are generally without root hairs; in other 

 cases it penetrates the cortical tissue of the root itself, 

 which may also be furnished with root hairs. Accord- 

 ing to the researches of Frank, the fungus of the 

 mycorhiza lives in symbiosis with the higher plant, 

 attacking the humus and also the mineral resources of 

 the soil, and passing on the food thus obtained to the 

 host plant. In some few cases the host plant possesses 

 no green assimilating leaves, and is wholly dependent 

 upon mycorhiza to obtain its necessary carbon from 

 the humus. Such a case is seen in the Neottia Nidus- 

 avis, or Birds' Nest Orchis, to be found chiefly amongst 

 beech underwood in this country. 



More generally, the host plant is capable of nutri- 

 tion in the ordinary way when growing in media in 

 which nutrient salts are abundant, but becomes myco- 

 trophic in soils and situations unfavourable to the pro- 

 duction of directly absorbable food — as, for example, in 

 heaths and moors, where the soil is almost wholly 

 humus, or beneath the shade of trees, where nitrates are 

 rarely found and where illumination is insufficient for 

 much assimilation. Later researches, particularly those 

 of Stahl, have shown that the symbiosis of mycorhiza, 

 instead of being a phenomenon restricted to a few 

 species, is widely diffused among many classes of plants, 

 and is indeed causally connected with other facts of wide 

 general importance in plant nutrition. It has already 

 been indicated that the cultivated plants give off con- 

 siderable quantities of water by transpiration ; the form 

 and arrangement of their leaves are adapted to expose 



