IRREGULAR NUTRITION 



233 



almost devoid of chlorophyll, are, like Monotropa and Sarcodes, 



obviously incapable of normal plant nutrition. The general view, 



which is not entirely satisfactory, is that 



during the course of evolution the plants 



have come to rely chiefly on organic food 



secured from the mycorrhizal fungus ; and 



this in turn derives its supply from the 



abundant humus of the soil which these 



plants inhabit. 



It is, however, probable that in the 



seedling stage all Orchids, green or sapro- 

 phytic, are to some extent dependent on 



assistance derived from the mycorrhizal 



fungus. Observation shows that in the 



presence of the fungus, infection of the 



Orchid seedling frequently occurs at a very 



early stage (Fig. 161). The difficulty of 



securing germination of Orchid seeds in 



horticultural practice is well known. Ex- 

 periment has shown that usually Orchid 



seeds will not develop under sterile condi- 

 tions. Inoculation with the fungus is fol- 

 lowed by normal germination (Fig. 162). 



The explanation may be that the Orchid 



seed requires a stimulus before germination 



will take place ; the stimulating substance, 



and additional food, may be provided 



by the fungus. The nutritional aspect 



is supported by experiments which show that if Orchid seeds 



are provided with organic 

 food such as sugars, normal 

 development will proceed in 

 the absence of the fungus. 

 Both of these observations 

 have been put to practical 

 application in horticulture, 

 and Orchid plants can be 

 raised (a) by sowing seeds 

 on special soil infected 

 with the mycorrhizal fun- 

 gus, or (b) by feeding the seedlings with organic materials. The 



Fig. 160. 



Lower part of a plant of Neottia 

 nidus-avis, showing the dense mass 

 of roots springing from a central 

 rhizome, which is concealed by the 

 roots. s=scale-leaf. /=flower. 

 r =root. ( x §.) 



Fig. 161. 

 Seed of Neottia infected by fungus at a very early 

 stage in germination. The outer line represents the 

 testa. (After Bernard.) ( x ioo.) 



