PSALLIOTA CAMPESTRIS 397 



conditions beneath the surface of the soil are not favourable 

 thereto. The first reaction of the subterranean mycelium appears 

 to be the formation of a network of cord-like strands. Probably 

 these strands serve as a reservoir for the protoplasm and other 

 materials which are to be put into the fruit-bodies ; and doubtless 

 they are produced at the expense of substances supplied by the 

 distant and actively-growing hyphae which are still attacking the 

 substratum and extracting from it its nutriment. Certain of these 

 strands, during their formation, grow upwards until they reach 

 the surface of the soil and, as we know from observation upon 

 artificial mushroom-beds made of soil-covered horse manure, can 

 penetrate through an inch or more of non-nutritious compact soil 

 or even through sand. This upward growth of mycelial strands 

 absorbs both material and energy, but it is absolutely necessary 

 in order that the reproductive bodies may be formed at the surface 

 of the ground, where alone the spores may be liberated so as to 

 secure their dispersal by the wind. One may ask whether or not 

 the upward direction of growth of some of the strands is due to 

 negative geotropism. Now, on artificial beds having various 

 shapes with surfaces at the sides and bottom, mushrooms may 

 arise anywhere on the surface (Vol. I, Fig. 17, p. 51). I am there- 

 fore inclined to believe that the strands grow upwards to the surface 

 of the soil in fields not as a response to the stimulus of gravity but 

 for some other reason. It is not improbable that the upward 

 growth is purely a matter of chance. The mycelium, owing to 

 its wonderful power of branching and to the apical growth of its 

 hyphae, doubtless tends to penetrate through the interstices of the 

 soil in all directions of space. This being so, some of the hyphae 

 must make their way upwards. The strands are then formed 

 about these hyphae. After being formed the strands immediately 

 proceed to the formation of rudimentary fruit-bodies. 



Why should the rudimentary fruit-bodies never be formed any- 

 where else than at the surface of the soil ? We can only suppose 

 that this position is decided by the reaction of the mycelium to 

 one or more external stimuli or by the absence of mechanical 

 pressure. The stimuli most likely to be involved seem to be light, 

 moisture, or atmospheric gases. Now light is unnecessary for the 



