UPWARD PRESSURE OF THE STIPE 117 



that the stipe-base is always situated in darkened crevices of the 

 substratum and so that it never projects freely above the general upper 

 surface of the substratum. The significance of this arrangement for 

 the welfare of the fruit-body as a whole will be discussed in the 

 Section treating of the mechanical fixation of the fruit-body. 



The Relations of the Fruit-body with Gravity. — At its first 

 origin, a fruit-body appears to be ageotropic and, for a short time 

 afterwards, will grow outwards from its substratum in any direction 

 — upwards, downwards, or sideways — without bending. However, 

 as soon as the stipe-base has attained a length of a few millimetres 

 and the pileus has become differentiated, the sti.pe-base gradually 

 becomes sensitive to the stimulus of gravity and in consequence, 

 during its further growth, bends slowly upwards so that it becomes 

 curved. Finally, the stipe becomes very strongly negatively 

 geotropic and it then pushes the pileus vertically upwards (Figs. 61, 

 p. 109, and 66). The jnleus does not respond to any geotropic 

 stimulus : both its flesh and its gills are ageotropic. 



The Upward Pressure of the Stipe. — It was noticed in the dish- 

 cultures that, under the stimulus of gravity, a young pileus was 

 often pushed upwards by the stipe with considerable force. This 

 was made evident, sometimes by the raising of superincumbent 

 dung-balls, and at other times by the actual breaking of a dung-ball 

 into two or more pieces. It seemed of interest to measure the 

 pressure exerted by the stipe, and the following experiment was 

 therefore carried out. 



A young fruit-body (Fig. 67), consisting of a well-developed 

 basal bulb and a small pileus, was seated upon the bottom of a glass 

 culture-dish in an upright position. The bulb was 1-4 cm. high 

 and • 7 cm. wide just below the pileus, and the pileus had attained 

 a height of 1 • 4 cm. and a diameter of • 8 cm. A piece of a collecting 

 tube, 1 -4 cm. high and 1 -0 cm. internal diameter, with a flat end, 

 was then inverted over the enlarging pileus so as to provide it with 

 a cap ; but, before this cap was placed in position, some cotton- wool 

 was put in its interior so that, eventually, while the very top of the 

 pileus would press against the glass end, the rounded upper sides 

 of the pileus would press against the cotton-wool packing. After 

 the cap had been adjusted, a brass weight of 20 grams was placed 



