Lecture X —145 — Obligate Symbiosis 



Impotence of Unaided Small Seeds: — Ceillier (1912) pre- 

 sented a suggestive paper on the germination of small seeds. In 

 certain cases, such as in Juncaceae, the seeds are small and little 

 differentiated but as they possess chlorophyll they are able to begin 

 photosynthesis immediately on sowing. Small seeds with much re- 

 duced embryO' also occur in parasitic forms such as Cuscuta, Oro- 

 hanche, etc. ; no fungus is present in these genera but apparently 

 germination is not successful unless contact is made with organs of 

 the requisite host. It may be, said Ceillier, that the stimulus neces- 

 sary in these cases is analagous to that requisite to bring about root- 

 formation in those plants with "obligate mycorrhizae". Here is a sug- 

 gestion of interest : Cuscuta can germinate unaided but only as it 

 develops parasitic connections can it be nourished. By an apparently 

 casual meeting with a host is its vigorous life assured. So it is ap- 

 parently with the orchids ; they have no specific fungi (as stated in 

 some detail by Curtis, 1939; also by Derx, 1937) ; but without any 

 fungi at all they appear as helpless as Cuscuta without an host. This 

 is more or less the conclusion of Downie (1940) who used Knudson's 

 methods with Goodyera repens and stated that "The experiments . . . 

 tend to support the original contention of Bernard that endophytic 

 fungi play a large, if not the whole, part in germination of orchid 

 seeds in the field." 



Germination of fungus-spores, seeds and pollen-grains has been 

 shown to depend on external supplies of activators, dependence on a 

 particular activator being due to a failure to synthesize this or a 

 similar substance (Brown, Nature 157:65-69, 1946).* 



Carbon Supply to Embryo: — Another angle of the problem 

 was presented by Pollacci & Tredici (1936) : It is not the fungus 

 per se that causes germination but something produced by the fungus. 

 Using seeds of Cymbidium, Cattlyea and Phalcnopsis, he found that 

 germination was accomplished by filtrate from Rhizoctonia; also from 

 mycelium isolated from Phalcnopsis; and perfect plants were grown 

 to flowering age. What these substances are that produce germination 

 remain to be discovered; but Beau (1920) suggests that they are 

 carbonaceous. He grew the orchid fungus on a gelatine to which 

 salep was added and placed seeds of Spiranthes and Orchis on the 

 glass but not in contact with the gelatine. When hyphae of the fungus 

 grew up and into the seeds, germination was accomplished and growth 

 continued until the hyphae were destroyed that connected the seed 



*ScHAFFSTEiN "insists that seeds of the genera Vanda and Phalenopsis, for 

 germination, must be supplied with the vitamin, vandophytin, which they lack" 

 (Jahrb.wiss.Bot. 68:720-752, 1938). 



