IRREGULAR NUTRITION 



231 



which has been compared with phagocytosis in animals, is perhaps 

 to be regarded as a method of keeping the fungus in check, its products 

 may be of use to the plant as nutrient substances. A proportion 

 of these food substances may have been originally derived from the 

 Heather itself but some part may have been obtained by the fungus 

 from the humus of the soil, and by digestion this is placed at the disposal 

 of the plant. In addition this particular fungus may, like the root- 

 nodule organism, have the faculty of fixing atmospheric nitrogen 

 (see p. 235). Any nitrogen so fixed would be transferred to the host 

 by digestion. There is no evidence that food materials obtained 

 from the fungus are essential to the existence of the adult Heather 



Fig. 158. 

 A single superficial cell of the young root of Common Heather {Calluna vulgaris) 

 showing the endotrophic fungus, and its penetration of the cell-walls. (After 

 Rayner.) ( x 1500.) 



plant, though it is possible that the plant is thereby enabled to thrive 

 better in the rather unfavourable habitats which it frequents. Some 

 experiments have indicated that the association with the fungus is 

 beneficial or even essential to the proper development of seedling 

 Heathers, but this point is not finally settled. A special feature of 

 the mycorrhiza of Heather is that the fungus is not confined to the 

 roots, but is to be found also in the aerial organs. 



In the Orchids, we find that while the majority are green and 

 normal in appearance, a number are complete saprophytes and show 

 the usual reduced vegetative system. The mycorrhizal fungus, 

 known as Rhizoctonia, inhabits the cortical cells of Orchid roots, 

 and frequently of the tubers that characterise many Orchids. These 

 internal hyphae are connected up to a certain extent with other 

 hyphae growing in the soil. In some of the infected cells the fungus 

 soon undergoes digestion, as in Heather ; while in others, termed 

 Host cells, the fungus persists unharmed, at least for a considerable 

 period (Fig. 159). These two types of cell (digestive and host) may 

 be very regularly arranged, a well-known example being provided 



