Lecture X 



143 Obligate Symbiosis 



lated seed germination of the same species. The same sort of relation- 

 ship was supposed by Wibiral, 1910, to hold good with respect to the 

 Gentians, 



Hydrolysis of Starch: — In germination of an orchid seed 

 (BuRGEFF, 1910), at first some oil is transformed into starch; then 

 the fungus grows in by way of dead suspensor cells, changes starch 

 to sugar and thus increases osmotic energy and greatly accelerates 

 growth. The fungus is now digested by enzymes produced by the 

 orchid cell and a clot is left. With most orchid seeds, said Burgeff, 

 there is never germination without a fungus. 



Sugars and Asymbiotic Germination: — In stating that the 

 fungus changes starch to sugar another major thesis was enunciated. 

 Bernard had initiated asymbiotic orchid seed germination by causing 

 the seed to germinate asymbiotically with salep, a product of dried 

 orchid tubers rich in bassorin, a polysaccharide (cf. Ballion, 1924). 

 Beau (1920) had further experimented with seeds of Spiranthes and 

 Orchis, causing them to- germinate by transfer of salep material from 

 gelatine to seed via a fungus. Ballion & Ballion (1924) secured 

 strictly asymbiotic germination of Cattlyea with a medium half mineral 

 and half organic. Knudson (1922) extended these experiments by 

 trying the effect of various sugars on germination of orchid seeds. 

 He sterilised seeds in calcium hypochlorite and cultured them on agar 

 slants; and found that seeds of Cattlyea, Laelia and related forms 

 germinate when sugar is supplied, fructose appearing more favourable 

 than glucose. In glucose, chlorosis resulted, the concentration of 

 glucose appearing important. Later (1924) he germinated seeds of 

 various hybrids of Cymhidium, Odontoglossum, Phalenopsis and 

 Ophyrs, finding "almost 100%" germination in sugar solution with- 

 out fungal aid. He said: "These experiments lend further support 

 to the hypothesis that the germination of orchid seed is dependent on 

 an outside source of organic matter." Failure to germinate without 

 sugar suggests necessity of an internal factor to activate chlorophyll, as 

 indicated by Briggs. After the process is initiated, then seedlings 

 develop without aid either of fungus or of sugar. In still further 

 experiments, Knudson (1925) isolated orchid fungus resembling 

 Rhisoctonia repens from Cattlyea, Cypripedium and Epipactis; and 

 grew it in culture solution with added "0.5% starch". This fungus 

 induced germination in Cattlyea "but not 100%", but there was no 

 germination without it. The h.i.c. was increased by the fungus due 

 tO' organic acids excreted by it, best growth being between pH 4.7-5.2. 

 I'lie fungus digests starch and changes it to sugar, while some of the 



