252 THE BIOLOGY OF FLOWERING PLANTS 



end. At this stage the little plant is only 3 to 4 mm. long. 

 Then a bud with one or two leaf rudiments appears at the 

 upper end (Fig. 32). 



This stage is never reached unless infection by the 

 symbiotic fungus takes place from the soil. Bernard 

 followed the mode of infection with sterilised seeds grown 

 in association with a fungus isolated from mature orchid 

 roots. In one case, Bletilla hyacinthina, germination pro- 

 ceeded to the leaf stage without the fungus, but no roots 

 were formed. In another, a hybrid of a Cattleya with a 

 LaeHa, germination proceeded for three months, to a point 

 at which the tuber possessed a little green ring above and a 

 few hairs below ; degeneration then set in unless infection 

 took place. In a third case, a Cypripedium hybrid, no 

 germination occurred unless the embryo became infected at 

 the start. Burgeff obtained similar results. Recently an 

 American investigator, Knudson (1922), has obtained ger- 

 mination of a hybrid Laelia- Cattleya on substrata containing 

 sugar, without the presence of the fungus ; confirmation 

 of this result will be awaited with interest. Bernard's 

 results showed that there was a varying degree of necessity 

 for the presence of the fungus, and wider examination may 

 show still greater differences of behaviour. 



Bernard believed that there was a high degree of speci- 

 ficity in the fungus. In experiments with the seeds of a 

 Phalsenopsis he found that normal germination occurred 

 with the fungus isolated from Phalaenopsis roots ; with a 

 Cattleya fungus the seeds were infected but the fungus 

 killed the seeds ; while with an Odontoglossum fungus infec- 

 tion took place and germination started, but the fungus 

 was soon totally digested by the plant, and no further 

 development took place. Other species were less specialised. 

 Burgeff tested the germination of the seeds of a Laelia- 

 Cattleya hybrid with fungi isolated from seventeen different 

 orchids, many of them European ; in four cases germination 

 was normal, in others it proceeded to various stages, but 

 the fungi were either too strong, invading the embryo 

 too vigorously, or too weak, undergoing digestion by it. 



