278 PRINCIPLES OF SOIL MICROBIOLOGY 



apogamous oospores or sporangia, which remain latent for a long time 

 and mature their spores only when environmental conditions are favor- 

 able. By means of these spores the organism spreads through the soil. 

 The mycelium forms a network in the soil, surrounding the root system 

 of the plant and growing from one plant to another. The phycomycete 

 leads both a saprophytic and endophytic existence. The endophytes of 

 orchids have no similarity to these endophytic phycomycetes, but are 

 true Mycomycetes, perhaps Basidiomycetes. In addition to the endo- 

 phytic phycomycete, most plants show that the mycelium of the latter is 

 overgrown with the endophyte of the orchid mycorrhiza fungus. This 

 form belongs to Rhizoctonia solani Kiihn or Moniliopsis aderholdi Ri'ihl, 

 among the primitive basidiomycetes (Hypochnus group). Peyronel 

 also found, in addition to these two endophytes, a certain number of 

 saprophytic fungi living at the expense of weak or dead tissues in 

 nearly all plants studied; these fungi belong to the genera Pythium, 

 Fusarium, Dydymopsis and Rhizomyxa. Asterocystis radicis was found 

 not only on the dying roots, but also regularly on those having a normal 

 appearance, and can also be classed as a mycorrhiza fungus. Out of 

 150 species of phaenerogams growing in different localities and under 

 different environmental conditions, Peyronel found the Rhizoctonia 

 type in 135. The phycomycetic mycelium, with which the Rhizoctonia 

 is usually associated, develops before the Rhizoctonia, thus facilitating 

 the penetration of the latter and making the host plant more receptive. 

 Melin 167 differentiated between true mycorrhiza and pseudo-mycor- 

 rhiza in forest trees. The fungi of the former belong to the Hymeno- 

 mycetes, including various species of Boletus, Amanita and Tricholoma, 

 Lactarius, Cortinarius, Russula, some being more specialized than others. 

 These form mycorrhiza with pine trees, birches, poplars, and other 

 forest trees. Pseudo-mycorrhizas are formed by a number of com- 

 mon soil fungi (Mucor, Verticillium), which can penetrate the roots, 

 when they are not infected by the true mycorrhiza; Mycelium radicis 

 atrovirens (probably Cladosporiwn humifaciens) belongs to the latter 

 group. Penicillia are indifferent to the roots. There is a competition 

 in forest soils between the different fungi, in attacking the roots of the 

 trees; the true mycorrhiza formers, when present, will enter the root 

 first. The true mycorrhiza fungi (species of Boletus) grow best at pll 



167 Melin, E. Experimentelle Untersuchungen iiber die Birken und Espen- 

 mykorrhizen und ihre Pilzsymbionten. Svensk. Bot. Tidskr., 17: 479-520. 

 1923. 



