174 I^IFE : OUTLINES OF GENERAL BIOLOGY 



for each kind of Orchid. These fungi are normally abundant in the 

 tropical soil; and as the orchid seeds are very small, very numerous, 

 and wind-borne, there is little risk of their not finding suitable 

 fungi in the orchid's native haunts. But it is far otherwise when a 

 new orchid is brought to a new hothouse in Europe. 



If the seeds of an orchid be so\\ti on a sterilised medium, which 

 ordinary seeds would utilise, there may be slight swelling, but there 

 may be no germination or no more than a beginning. If the appro- 

 priate fungus is supplied, it penetrates into the cells of the orchid 

 embrj'o, and all proceeds normally, except that the development is 

 in most cases very slow. When inappropriate fungus hyphae are 

 supplied, they may induce germination, but in most cases this soon 

 comes to a standstill. This may be because the fungus is too strong 

 for the orchid embryo and parasitises it, or because the orchid 

 cmbrj^o is too strong for the fungus and digests it. It should be 

 noticed that the seeds of some kinds of orchid can be stimulated and 

 partnered by one out of several species of fungus; but Bernard 

 convinced himself that the most highly evolved orchids were the 

 most fastidious, so to speak, in regard to their partners; and it Is 

 generally agreed that the converse proposition is true, that the most 

 generalised fungi can enter into partnership with several different 

 kinds of lower orchid. 



Recent experiments by Professor Knudson of Cornell University 

 indicate that the indispensability of the orchid partner-fungi has 

 been exaggerated. Certain orchids can flourish and flower without 

 partners; and a little sugar in the culture in which the minute 

 seeds are sown may serve to induce germination and the growth of 

 vigorous scedlin|:;s. 



We cannot go into the details of a fascinating story, but it is 

 important to notice some of the grades of the partnership. Thus the 

 seeds of the Lady's vSlipper Orchis (Cypripedium) will not germinate 

 at all without fungus contact; in Cattleya, very common in green- 

 houses, germination occurs without the presence of the fungus, but 

 the seedling is very short-lived; in Blctia the seedling can live 

 independently for some months, but after that, in the absence of its 

 natural partners, it stops growing. 



As to the physiological nature of the symbiosis, only tentative 

 statements can Ix^ made. First, as regards germination, it may 

 be that the fungus supplies a ferment which stimulates; but it 

 must also be noted that the seeds of orchids are poorly equipped 

 with reserves, and the fungus may be of service in establishiiig a 

 very early nutritive connection with the soil. It is interesting to 

 recall the fact that the spores of the "Stag's Horn Moss" (Lyco- 

 ixxlium) do not develop beyond the 5-cell stages in cultures that 

 are devoid of fungi. 



But what of the mature plant? It is probable that some of the 



