Kelley — 144 — Mycotrophy 



sugar is changed to organic acid. With less starch, most of the seeds 

 were killed by the fungus. With starch in the culture solution there 

 is no germination unless fungus is supplied. With sugar and fungus, 

 plants made much better growth, the beneficial effect being attributed 

 to the fact that the fungus changes the h.i.c. to a more favourable con- 

 centration. La Garde (1929) found maltose the most efficacious sugar, 

 and the best h.i.c. between 4.8 and 5.2, no germination taking place 

 above pH 6.0. Clement (1924&) found that for Odontoglossum 

 the pYi was best held at 6.5-6.8, which he said possibly influenced 

 solution of phosphates. For Goodyera, Downie (1940) said that 

 levulose and dextrose are better than sucrose. But the same author 

 said that seeds of Goodyera on mineral nutrient solutions adjusted 

 to range of 3.6-7.6 failed to germinate without the fungus, but with 

 the fungus no other aid was necessary. And Porter (1942) found 

 the results of symbiotic tests were generally far superior to those 

 secured on the asymbiotic substratum. 



Fungus Supplies Sugar: — These experiments of Knudson's 

 confirm earlier discoveries and further elucidate the fungus-orchid 

 relation. It seems evident that the crucial action in germination of an 

 orchid seed is provision of that seed with a sugar solution, although 

 recent work indicates that something more than a carbohydrate is 

 necessary for germination and development of the orchid, namely 

 a "growth-factor" which is supplied by the fungus. Furthermore, 

 Curtis (1943) thinks that vernalization is necessary for germination 

 of Cypripediiim seed: he secured some (20%) germination in an 

 adjusted environment with a covering of agar to prevent a down- 

 ward diffusion of oxygen. But provision of sugar can come only 

 through aid of a fungus, or by artificial supply by man : if there 

 is a third alternative we do not know it. Apparently the orchid 

 seed is incapable of supplying its need through any autolytic action 

 since practically all orchid seeds fail tO' germinate unless stimulated 

 by an external aid as so early indicated by Bernard. Knudson fails 

 to meet this situation when he asserts that fungi are not responsible 

 for germination of orchid seeds ; and his only suggestion is that in 

 nature other fungi than the "orchid fungi" may provide the seed 

 with sugar. But what other fungi ? 



Downie (1943) suggested that the symbiotic fungus of Good- 

 yera repens, which cannot survive in compact humus, may live in 

 other ways; for he found it on bi foliar spurs of Piiius syh'estris 

 before the spurs were shed. He thought that his discoveries "contra- 

 dict Knudson's hypothesis that orchid seed germination under 

 natural conditions is effected by the action of non-symbiotic sapro^ 

 phytic fungi on the organic substratum". 



