IRREGULAR NUTRITION 



201 



arising from either stem or root. Through these tlie hyphal filaments 

 pass, either outwards to estabhsli connection with the humous soil, or 

 inwards to cause new infections. (Fig. 1 51, p.) 



Within the Orchid family there is some variety in the appHcation of this 

 indirect nutrition. In some cases (Bletiella), germination is independent 

 of tlie infection of the seedling, which need not occur till several leaves have 

 been formed. In the Ophrydeae, which include most of the native ground 

 Orchids with tuberous stocks, the infection is very carlj'. It causes 

 that tuberous swelling of the sympodial stock and roots, which is repeated 

 in each annual growth of these plants (Fig. 152). Most of the Orchids 

 that are thus mycorhizic possess chlorophyll, giving them a more or less full 

 green colour. They are thus only partially dependent upon the symbiosis. 



Fig. 152. 

 CtLi-iiiinatiou of the Ophrydeae. A =young tuber of Orchis in section. B^ >ounK 

 plant. C = youiig plant of Ophrys in section. D = diagrani of young plant of 

 Platanlhera, second year ; /) = tuber of first year; I, /^succeeding tubers ; <? — scales ; 

 ; = infected region : r — root, (.\fter Bernard.) 



But others are whollv dependent, growing as complete saprophytes. The 

 most important native examples are Corallorhiza and Neottia. The former 

 grows in Pine woods with its rhizome embedded in rich humous soil. It 

 consists of freely branched underground shoots, bearing scale-leaves, but no 

 roots. The aerial part of the plant is a simple scape, bearing only colourless 

 scales, and at the top a raceme of small pale flowers. The fungus gains 

 access to the tissues through the numerous tufts of hairs scattered over the 

 surface of the shoot. Digestion of the hyphae has been observed, and as 

 they disappear starch-grains are formed in the inner tissues. 



The relation of host and fungus has been worked out very fully in the 

 Bird's Nest Orcliis (Neottia), which grows commonly in woods, putting up its 

 brown flowering scape in early summer. If it be dug up, the brown " bird's 

 nest " is found to consist of thick fleshy roots crowded upon a short central 

 axis which bears scale leaves, with lateral buds in their axils. It is, in fact, 

 a rhizome, as in Corallorhiza ; but whereas roots are absent here, in Neottia 

 they form the most prominent feature (Fig. 153 C, D). 



