BOTANY OF THE LIVING PLANT 



otherwise grow below soil level (p. 459), and it has been shown that 

 it is tl ame fungi that participate in the mycorrhiza of the roots oi 



tree. A good deal of work has been carried out with a view to 

 determining the importance of ectotrophic mycorrhiza to trees, 

 chiefly in connection with afforestation schemes. There is evidence 

 thai in some types of soil the tree does not flourish unless the mycor- 

 rhizal association is properly developed, though the explanation of this 

 is not yet clear. Benefit to the tree might arise as already mentioned 

 by absorption of organic food derived by the fungus from the humus 

 of the soil, and subsequently transferred to the tree. A further sug- 

 gestion is that the fungus may supply water and mineral salts more 

 rapidly than the tree could absorb these for itself. 



Whether the fungus gains much from the association with the 

 higher plant is uncertain, especially where the latter is of the com- 

 pletely saprophyte class, such as Monotropa or Sarcodes ; but there is 

 evidence that in some cases the fungi of tree-mycorrhiza develop 

 unsatisfactorily apart from their normal associate. 



(b) Endotrophic Mycorrhiza. 



In this type of mycorrhiza the fungal filaments penetrate and 

 inhabit the cells of the higher plant and come into intimate relation 

 with their protoplasts, though normally these do not appear to be 

 adversely affected. Besides the Orchids and Heaths endotrophic 

 mycorrhiza is present in many other flowering plants and in some 

 lower plants, for example in Lycopods, and certain Liverworts. As 

 with the ectotrophic type, most of the plants with endotrophic 

 mycorrhiza are normal in appearance ; but some, such as the Bird's 

 Nest Orchid (Neottia), are almost devoid of chlorophyll and are 

 classed as complete saprophytes, being obviously of irregular nutrition. 



Special attention has been given to the endotrophic mycorrhiza 

 of the Heath family and of the Orchids. The fungus concerned in the 

 mycorrhiza of the common Heather [Calluna) has been identified as 

 a species of the fungal genus Phoma. The fungus enters the fine 

 young roots of the Heather in spring, from the soil, and is to be 

 found within the cortical cells, these internal hyphae being connected 

 with others growing on and near the surface of the root (Fig. 158). 

 The existence of the fungal filaments within the cells of the root 

 is of limited duration, for during the summer and autumn the filaments 

 undergo what appears to be a process of digestion under the influence 

 of enzymes of the Heather plant. While this digestive process, 



