I 



Lecture VI — 81 — Environment 



rhizae. Naturally under optimum light exposure photosynthesis is 

 carried on most favourably and the host is accordingly better pro- 

 vided with assimilate. Bjorkman finds a connection between assimi- 

 lation and mycorrhizal formation, for which the reason is given in the 

 last chapter of this book. 



Phenology: — With reference to seasonal aspects of mycorrhizae 

 there are a number of observations made incidentally in the course of 

 other studies. Early in the history of our science, R. Hartig (1886), 

 who was no friend to the concept of mycotrophy, asserted that tree 

 roots are free from fungi in summer and that mycorrhizae are present 

 only in autumn and winter; while McDougall (1914), upon whom 

 Hartig's mantle has to some degree fallen, claimed that mycorrhizae 

 are annual, being formed in summer and persisting through the winter. 

 A. B. Frank (1888) countered Hartig's statement by saying that 

 "die Mykorhiza zu keiner Jahreszeit ihren Pilzmantel verliert". The 

 mycorrhiza, he said, is formed in the earliest youth of the plant and, 

 like all absorbing roots, dies when it has exhausted its soil locus. My- 

 corrhizae can exist at least two years, probably much longer. These 

 statements were an amplification of his 1885 assertion that mycor- 

 rhizae have a limited life-span, some being lost while others are being 

 replaced ; and it seems evident that mycorrhizae are able to live several 

 years. Moller confirmed Frank by stating ( 1890) : "Als Beweis 

 dafiir fiihre ich an, das ich bei Material, welches im Januar gesammelt 

 war, gleichwie bei solchem im Juni sammtliche Entwicklungszustande 

 und in gleicher Verteilung gefunden habe". The fungus grew out 

 simultaneously with the tuberous mycorrhiza of the pine studied, in 

 summer rapidly but at other seasons as the cold permitted. 



For the rebuttal, McDougall & Jacobs (1927) stated that at 

 7,100' A.T. on Mt. Logan, Utah, only dead mycorrhizae of the pre- 

 ceding year were found on Pseudotsnga inncronata. Above 7,000' 

 on Mt. Logan and at 10,000' on Mt. Washburn in the Yellowstone 

 Park, only dead mycorrhizae were found. New mycorrhizae can be 

 formed only when new rootlets are being developed and mycorrhizal 

 fungi are active, and these conditions seem to obtain in the latter part 

 of the growing season. 



It will be recalled that Busgen (1901) did some cultural work on 

 ash, beech, maple, oak, and spruce, to learn more of the disputed ques- 

 tion of periodicity of root growth. He found that in Germany best 

 growth occurs in June and October with little growth occurring in 

 July and August. In March there are numerous roots growing, also 

 in November and December. In conifers a winter rest is indicated by 

 browning of the root-tips. Goebel, in the Organography, calls at- 



