SOIL FUNGI 273 



hyphae in the epidermis and in the outer cortical layers are intracellular 

 in nature, but, with horizontal spreading of the mycelium in the second 

 root layer, branching takes place, some of the branches penetrating the 

 cell wall into the intracellular spaces. Vesicles are formed in the second 

 region; they are looked upon as arrested sporangial developments or as 

 spore bearing bodies which may function as temporary reserve organs 

 during the early stages of the invasion of the fungus. Arbuscles and 

 sporangioles are formed in clusters in the third region, the middle cortex. 



Plants forming mycorrhiza. A large number of plants, including 

 perennials and annuals, are capable of forming mycorrhiza with different 

 fungi. Wild plants and fruit trees form mycorrhiza more readily than 

 cultivated plants. Some fungi can attack a large number of plants, 

 whereas some plants can form mycorrhiza with different fungi. This 

 phenomenon is especially highly developed in certain cases, as in the 

 orchids and Ericaceae; in other words, there is no direct specificity 

 between the plant and the infecting fungus. Stahl 143 and others demon- 

 strated that most higher plants, with the exception of submerged 

 water plants and certain specific large families of plants (Cruciferae, 

 Cyperaceae and Polypodiaceae), possess always or occasionally my- 

 corrhiza formations. Plants with rapid transpiration can absorb their 

 mineral food material without the fungi, whereas plants with weak 

 transpiration can obtain a sufficient supply of minerals only by the 

 assistance of the symbiotic fungi. Stahl's theories, however, were not 

 based on experimental evidence. 



Bernard 141 observed a series of transition stages in the degree of 

 dependence of the plant upon the fungus. In case of a few species 

 of orchids, as Bletitta, the seeds germinate but are unable to form roots, 

 without infection; in the majority of other orchids, the embryo is arrested 

 at a much earlier stage. The degree of specificity between plant and 

 fungus varies, as does the resistance of the plant to the fungus invasion. 

 Bernard demonstrated experimentally that symbiosis between the 

 plant and fungus is obligate. The results obtained by practical orchid 

 growers confirmed these facts. N. Bernard and Magrou 144 developed 

 a theory of tuber formation, in the evolution of plants, based on the 



143 Stahl, E. Der Sinn der Mykorrhizenbildung. Jahrb. wiss. Bot., 34: 539- 

 668. 1900; Stickstoffbindung durch Pilze bei gleichzeitiger Ernahrung mit 

 gebundenen Stickstoff. Jahrb. wiss. Bot., 49: 579-615. 1911. 



144 Magrou, J. Symbiose et tuberisation. Ann. Sci. Nat. (10), 3: 181-275. 

 1921; La symbiose chez les hepatiques le Pellia epiphylla et son champignon 

 commensal. Ibid. (10), 7: 725-278. 



