ORCHID MYCORHIZA 251 



panied by characteristic changes in the nucleus of the host 

 cell seemingly of the nature of an interrupted division. 



This highly specialised differentiation into digestive and 

 host cells is found in many, perhaps all, other orchids, but 

 the arrangement in definite layers is not usually so sharp ; 

 nor is the quite definite localisation of the fungus and its 

 regular occurrence in all the roots general. 



Magnus describes, too, the case of Orchis maculata which 

 is, he says, characteristic of the European green geophytic 

 orchids. In these old well-developed roots are frequently 

 found with no trace of fungus, while in other roots the 

 fungus occurs only locally. In the two external cortical 

 layers the cells are host cells, and further in digestive cells 

 predominate. Reinfection of digestive cells may take 

 place. In Listera sometimes the whole of the infected cells 

 carry out digestion. In Orchis the two-layered epiderm is 

 sparingly infected, and from it numerous branching hyphae 

 run out through the root-hairs into the soil. In some 

 species, e.g. Platanthera chlorantha and Orchis masciila, 

 the rhizome is free from fungus and the young roots are 

 infected from the soil through their root-hairs when they 

 are 3 cm. long. In the rhizome of Corallorhiza the 

 fungus occupies the external layers of the cortex as a host 

 region, and the middle layers are digestive ; numerous 

 hyphae pass into the soil. 



Germination o£ Orchid Seeds. — The germination of the 

 orchid seeds is closely related to the presence of the fungus. 

 In the investigation of this relation, the first of the kind 

 known, Bernard was the pioneer. The orchid seed, e.g., of 

 Phalaenopsis, is minute, about a fourth of a millimetre long, 

 covered by a loose coat, and consisting of an undifferentiated 

 embryo of a few hundred cells. Its germination is peculiar. 

 It swells into a little spherical body, a tiny tuber, from the 

 lower end of which absorbing hairs grow out, while the 

 upper end becomes green. (The tuberous orchids of our 

 meadows, however, remain colourless underground, leading 

 a saprophytic existence for several years.) Only after four to 

 five months' growth does the first root appear at the lower 



