Lecture VI — 83 — Environment 



they had achieved renewed growth at the beginning of the growing 

 season by bursting through the fungal sheath. With Pennsylvania 

 trees and shrubs, mycorrhizae are present every month of the year 

 but particularly in late summer and autumn (Henry, 1933). On 

 deciduous trees of Scotland mycorrhizae are present every month from 

 November to March (Gordon, 1936). Still other observations are 

 to be recorded : Pecan mycorrhizae are to be found at all seasons in 

 Georgia (Woodroof, 1933) ; in Vitis it is a mistake to speak of the 

 dying of all roots in autumn, for only those formed in spring die while 

 those formed in autumn persist through the winter into spring (Rives, 

 1923) ; in Citrus seasonal variation was found, mycorrhizae being 

 best developed in the spring growing season (Reed & Fremont, 

 1935). In beech in England, the time of most rapid root growth 

 (chiefly spring and autumn) is marked by appearance of numerous un- 

 infected roots (Harley, 1937). This period is followed by one of in- 

 fection of the new roots. The shallowest chalk escarpment soils are 

 characterized by a short spring period of growth and infection ; in 

 deepest escarpment soils the spring growth persists longer, root growth 

 and infection going on together and being interrupted only by drought. 

 Infection is never complete and many uninfected roots are present in 

 spring and summer. In very acid plateau soils, roots are formed near 

 the surface and growth occurs in an upward direction in spring, incom- 

 pletely decayed litter of the previous autumn being colonized by un- 

 infected roots. In April and May infection takes place rapidly, while 

 in early summer it is nearly complete. 



For herbaceous plants there are various reports. In liverworts the 

 fungus was found occurring in autum (Schacht, 1854) ; in Pyrola 

 Stahl (1900) found mycorrhizae also in autumn but not in spring; 

 while Endrigkeit (1937) said that plants of Convallaria and Maian- 

 themum are almost completely fungus-free in spring. Orchid roots 

 collected in September were uninfected (Costantin, 1926) while 

 Beau (1913) stated that roots formed in Spiranthes at end of the 

 flowering season are infected from the soil but not from old roots. In 

 Galeola the symbiont invades the cortex during summer and autumn 

 and ingestion proceeds through the winter until the following summer 

 (Ham ADA, 1939). 



Cromer (1935) had noticed that mycorrhizae of Finns radiata 

 renew their growth after rain. According to Paulson (1924) dur- 

 ing drought of even short duration mycorrhizae are desiccated and 

 thereby killed. "Mycorrhiza does not revive after being destroyed by 

 lack of moisture and does not reappear on the return of copious rain 

 until new rootlets have been developed and they in their turn have be- 

 come associated with a fungus. . . . Observation of roots after heavy 



