274 PRINCIPLES OF SOIL MICROBIOLOGY 



symbiotic action of the plants with the fungi. The establishment of 

 symbiosis in potatoes and Orobus tuberosus germinating from seed brings 

 about the formation of tubers out of the buds at the base of the stem. 

 When the plants are kept from becoming infected with the symbiotic 

 fungi or do not establish the usual symbiotic relation, without otherwise 

 changing the conditions of growth, the same buds change into thin 

 stems, without the formation of tubers (No. 116, PI. XV). This theory, 

 however, is still in the theoretical stage, since the evidence is incomplete. 

 A relation between plant and fungus resembling that found in orchids 

 was also observed in Ericaceae, namely a complete dependence of the 

 seedling plant upon infection by the endophyte at a critical stage, 

 differing, however, in the mode of infection. 145 In the absence of infec- 

 tion, root formation is arrested and growth is finally inhibited. Infec- 

 tion of the primary root takes place soon after germination of the seed, 

 which is infected while still in the ovary. The formation by Arbutus 

 unedo of root tubercles, which are arrested secondary and successive 

 laterals of the season's growth, is due to the invasion by a fungus, which 

 at first acts ectotrophically, then as an endotroph of the peripheral 

 cells. 146 For the development of various forest trees, such as firs and 

 spruces, mycorrhiza are absolutely essential; 147 without them the plant 

 development will not be normal. A large number of annual and peren- 

 nial plants were also found 148 to form mycorrhiza; here are included 

 various plants among the Graminaceae (Zea 7?iays, Triticum sativum, 

 Hordeum vulgare, Avena saliva), Araceae, Liliaceae, Amaryllidaceae, 

 Iridaceae, Juglandaceae, Urticaceae, Lauraceae, Polygonaceae, Cheno- 

 podiaceae {Beta vulgaris), Portulacaceae, Violaceae, Cruciferae, Ranun- 

 culaceae, Saxifragaceae, Rosaceae, Leguminaceae {Genista tinctoria, 

 Medicago saliva, Melilotus officinalis, Trifolium pratense, Trifolium 

 repens, Ornithopus cornpressus) , Umbelliferae, Geraniaceae, Rutaceae, 



145 Rayner, M. C. Preliminary observations on the ecology of Calluna vul- 

 garis on the Wiltshire and Berkshire downs. The New Phytol., 10: 227-240. 

 1911 ; Obligate symbiosis in Calluna vulgaris. Ann. Bot., 29: 97-133; 1915; Mycor- 

 rhiza in the Ericaceae. Brit. Mycol. Soc, 8: 61-66. 1922. 



146 Rivett, M. F. The root tubercles of Arbutus unedo. Ann. Bot., 38: 661- 

 677. 1924. 



147 Melin, E. Experimented Untersuchungen fiber die Konstitution und 

 Okologie der Mykorrhizen von Pinus silvcstris L. und Picea abies (L.). Karst. 

 Mykol. Unters. u. Ber., 2: 73-335. 1923. 



us Peyronel, B. Prime ricerche sulle micorize endotrofiche e sulla mycoflora 

 radicicola normale delle fanerogame. Riv. Biol., 6: 463-485. 1923; 6: 17-53. 

 1924. 



