254 THE BIOLOGY OF FLOWERING PLANTS 



Bernard's discovery has already had important practical 

 applications. Orchid growers have long found it difficult 

 or impossible to germinate the seeds of many kinds. The 

 provision of the suitable fungus gets over the difficulty, and 

 the new method has already been applied by some growers 

 on a large scale (Costantin and Magrou, 1922). 



Bernard compares the action of the fungus on the plant 

 to a state of disease, " a benign disease " ; the reaction 

 of the plant is like phagocytosis. With the appropriate 

 fungus a nice balance exists, the attack is strictly limited. 

 If the fungus is too virulent it overcomes the defences of 

 the plant and becomes speedily and fatally parasitic. 

 Bernard found that prolonged culture on artificial media 

 made his fungi more virulent, but this has not been 

 confirmed by Burgeff. The parallelism of the relation to 

 that in Calluna is striking. 



The manner in which the fungus promotes the orchid 

 germination is not known, but reference must be made to 

 Knudson's view that it makes available for the seedling 

 insoluble carbohydrates in the culture media employed— 

 or naturally present in the humus. His cultures, in which 

 fungus-free germination took place, were carried out in 

 media with soluble carbohydrates, fructose being found 

 most favourable. The explanation seems too simple. 

 There are species in which germination proceeds to the 

 production of chlorophyll without the fungus and without 

 soluble sugars, and there stops. This is the case, too, with 

 Calluna. Now at this stage one must suppose the seedling 

 to be capable of assimilation, and the importance of a 

 transfer of carbohydrates is not obvious. 



Gastrodia.— One further case of special interest may be 

 described, that of Gastrodia elata, which inhabits oak woods 

 in Japan. It possesses a colourless, rootless tuber about 

 6 in. long with a corky covering, bearing a number of 

 small daughter tubers. The plant is a complete saprophyte. 

 The large tubers are infected by a species of a fungus, 

 Armillaria, strands of which spread over its surface and pass 

 into the soil, where they often bear their typical fructi- 



