68 NATURE OF MYCORRHIZA 



removed from the soil suitable fertilizers containing nitrogen, 

 phosphorus, lime, potash, etc., must be added to the soil to 

 replace the absorbed crude foods. 



The majority of plants do not absorb materials from the soil 

 in so direct a way as is illustrated in Fig. 29. They are de- 

 pendent for certain of their materials at least upon a low group 

 of plants termed fungi. These fungi more commonly consist of 

 delicate cobwebby threads (termed hyphae), such as are seen 

 on mouldy bread. This fungal group, known collectively as 

 mycorrhiza, comprises a number of forms or species, only a few 

 of which have as yet been identified. These delicate plants 

 spread through the soil and their hyphae enter either the outer 

 cells of the roots or form a mantle of more or less closely inter- 

 woven threads about them or the hyphae may sustain both of 

 these relations to the root. It has been demonstrated in many 

 instances that they transfer to the plant various substances 

 derived from the humus. Certain forms, probably a much larger 

 number than is now known, have the power to fix free nitrogen 

 much after the manner of the bacteria. It is also probable that 

 they sometimes take over the normal absorbing function of the 

 root hairs for frequently roots associated with Mycorrhiza develop 

 root hairs sparingly or not at all. This relationship is not 

 always one sided in its benefits for the fungus receives carbo- 

 hydrate material from the green plant. This state where two 

 or more plants live together is termed symbiosis. All stages in 

 the symbiotic relationship of the fungi and the green plants may 

 be seen. The green plant and the fungus may not be benefited at 

 all, a relationship termed commensalism. When the green plant 

 is injured by the fungus, this symbiotic relationship is termed para- 

 sitism. The symbionts may be equally benefited and not at all in- 

 jurious one to the other. The majority of our poplars, willows, 

 beeches, heaths, orchids and evergreens have attained this state 

 and do not flourish in soils where suitable fungi do not abound. 

 Perhaps this explains why it is so difficult to transplant certain 

 shrubs and trees. The fungi are easily injured and do not be- 

 come established in the new soil soon enough to keep the plant 

 alive. This mutual relationship has gone so far in some species 



