Lecture VI —75 — Environment 



that composed the forest were destroyed. Yet in the case of pine, 

 Cromer (1935) found that, while drought caused collapse of the 

 cortex of absorbing roots, it did not affect that of the mycorrhizae. 

 Besides physical dryness of the soil, physiological dryness must also 

 be considered; yet Faber (1925), in a study of volcanic soils of Java, 

 decided that occurrence of the same association in the crater of the 

 volcano as well as lower down on the sides in wet volcanic soil may 

 not be explained through the hypothesis of "physiological dryness" 

 of solfatara soil but it is the result of individual nutrient conditions on 

 young solfatara soils. 



Certain mycorrhizal plants such as Obolaria and Orchis spectahilis 

 are found only in moist shady places and thus indicate a relationship to 

 soil moisture. This relationship was noted as early as 1889 by Johow. 

 Dependence is probably on the mycorrhizal fungi which can extract 

 nutrient materials from moist humus and duff but not from dry mate- 

 rials. On the basis of soil moisture one might separate the mesic 

 mycorrhizal plants from the xeric ones such as ericads, conifers and 

 certain cupulifers, the former being chiefly endotrophs and the latter 

 ectotrophic. However, it must be noted that Voss & Ziegenspeck 

 (1929) have shown that xeromorphy in ericads may be due to physio- 

 logical conditions resulting from mycotrophy rather than to dryness 

 of soil. 



It must be observed that mycorrhizae are chiefly developed in the 

 uppermost A horizon in what is naturally the driest portion of the 

 soil, at least on well-drained sites. For this reason there is a selective 

 action on mycorrhizal plants, the mesic species being confined to sites 

 where minimum soil moisture for mycorrhizal development is higher 

 than for xeric species. Certain anomalies in plant distribution can 

 thus be explained: Thus, Pimis virginiana grows on hills of Penn- 

 sylvania and Maryland but is absent from adjacent sand flats of the 

 coastal plain of Delaware where oak woodlands flourish. It was found 

 by experiment that seedlings of the pine transplanted to open sands 

 of the Delaware area could not withstand desiccation of the summer 

 dry season ; but where pine is watered or grows in favourable lowland 

 (there is a colony of P. Taeda near Newark), the tree is able to grow 

 in spite of dry seasons. In the same way, Orchis spectahilis grows 

 on shaded north slopes of Pennsylvania woodlands but never on sunny 

 south slopes where xeric ectotrophs thrive. Boudier (1876) had 

 noticed that Elaphomyces is found on south and east slopes more than 

 on north and west slopes at Montmorency in France. 



Another fact that must be taken into account is that rainfall on an 

 area is by no means uniform in distribution nor regular in occurrence ; 

 and these irregularities have a consequent influence on mycorrhizal 



