54 RESEARCHES ON FUNGI 



that the pastures where Mushrooms grow are always more or less 

 horizontally disposed, large curvatures are quite unnecessary. As 

 a matter of fact, in extreme cases in the field, the needs of the 

 pileus can be fully met when the stipe bends through an angle of 

 only about 45 (Fig. 17, to the left). 



It seems probable that the position of the pileus at its origin is 

 simply determined by the direction of the stromatous strand, upon 

 the end of which the fruit-body concerned comes to be developed. 

 However this may be, there can be no doubt that, as soon as the 

 pileus and stipe have become differentiated, the direction of further 

 growth of the fruit-body (excluding mechanical resistance offered 

 by grass, &c.) is entirely controlled by the stimulus of gravity ; 

 and it is to this, and to this alone, that a Mushroom owes its 

 characteristic position in the field. The stipe, when not yet 2 cm. 

 long, becomes negatively geotropic in a zone just below the pileus 

 flesh; whilst subsequently the gills, after partial development, 

 place themselves very exactly in vertical planes by growing toward 

 the earth's centre. 



When Mushrooms come up in fields the mechanical resistance 

 offered by the herbage often hinders the stipe, when young, from 

 bending straight upwards. As the stipe gets longer and longer 

 the pileus becomes pushed more and more into freedom. When 

 this has been attained the stipe simply grows directly upwards, 

 but evidence of its early struggle is still left in its curved or zigzag 

 form (cf. Fig. 17). 



Werner Magnus 1 observed that when a Mushroom comes up 

 on a sloping bed so that it catches in the manure and has to push 

 up against it, the stipe becomes unusually long whilst the pileus 

 remains small. He also found that when a young pileus is pre- 

 vented from expanding by means of a ring of gypsum, its growth 

 practically ceases and the stipe attains a quite abnormal length. 

 This peculiar correlation in growth between pileus and stipe in 

 nature probably sometimes helps to determine the form of Mush- 

 rooms, for these occasionally are to be found pushing their way 



1 W. Magnus, "Ueber die Formbildung der Hutpilze," Archiv fur Biontologie, 

 Bd. I., 1906, p. 104. 



