L ecture VI -79- Environment 



noted deficient aeration and oxidation of peat soils affects distribu- 

 tion of mycorrhizae. This is all the more remarkable because work of 

 LuNDEGARDH and others would seem to indicate that soil atmosphere 

 might have a profound effect on the mycorrhiza and associated or- 

 ganisms, especially through an heightened CO2 content. The great 

 importance of optimum CO2 supply in tuber formation has already 

 been indicated by Molliard (1920), tubers failing to form in its 

 absence. It may be that in the future the soil air will be shown of more 

 importance to the mycorrhiza than some of the soil influences which 

 are now stressed. 



Soil Temperature : — As with soil air, there are no direct studies 

 on influence of soil temperature on mycorrhizae. But it is well known 

 that soil temperature does not fluctuate to the same extent as air 

 temperature ; and in woodlands where the soil is blanketed with a layer 

 of duff it is partially insulated from fluctuations of air temperature. 

 A woodland with heavy leaf litter is so well protected from frost that 

 the ground may not freeze all winter and in consequence the mycor- 

 rhizae are not destroyed as they often are on freezing. In contrast, 

 a woodland that lacks a protective leaf litter freezes and thaws re- 

 peatedly and only certain plants, especially deep-rooted ones, survive. 

 Again, through freezing and thawing many seedlings are heaved out 

 of the ground whereby certain species are prevented from establish- 

 ment in a habitat which would otherwise be suitable for them. Yet 

 freezing does not necessarily destroy mycorrhiza! plants for it has 

 already been seen that such occur in arctic and alpine situations. 

 Chaudhuri (1935) stated that the endophytes of hepatics studied 

 can withstand very low temperatures and even an exposure to 0° for 

 four weeks did not kill any of them. A great number of mycorrhizal 

 fungi seem benefitted by low temperature but in nature mycorrhizal 

 fungi exist at many varied temperatures (Melin, 1925). In the 

 Japanese orchid, Galeola, the fungus is dominant when its optimum 

 soil temperature of 25 °C prevails while the host is more active under 

 the more congenial conditions of the colder months (Hamada, 1939). 

 Yet high temperatures do not prevent mycorrhizal development, al- 

 though presence of mycorrhizae in the tropics does not necessarily 

 indicate toleration of high soil temperatures since these may be 

 comparatively low and steady in the rain-cooled, shaded forest. But 

 mycorrhizae of cactus are certainly exposed to extremes of soil 

 temperature and show the hardiness of the mycorrhizal association. 



The temperature of the soil may undoubtedly be changed by action 

 of microorganisms, and a soil with a rich microflora should be a 

 warmer soil and more favourable to winter survival of seedlings than 



