ADJUSTMENTS OF FRUIT-BODIES 51 



to look once more directly toward the earth. The rate and the 

 amount of the reaction depend upon various conditions, particularly 

 on the stage of development of the fruit-body and the amount 

 of the tilt. The younger the gills and the smaller the tilt, the 

 quicker and more complete is the readjustment hi a vertical 

 plane. An expanded Mushroom was placed so that the plane 

 of the pileus was vertical, and those gills which most nearly 

 occupied horizontal planes were then looked at edgewise with a 

 horizontal microscope. The reaction to the stimulus of gravity 



FlG. 17. Psattiota campestris. Adjustment of the pileus by geotropic curvature 

 of the stipe. A, Mushroom grown upside down in a pot. B, two Mushrooms 

 grown laterally in a pot. To the left, five Mushrooms gathered and sketched 

 in a field. All \ natural size. 



was found to begin in about an hour after the fruit-body had 

 been gathered and tilted. After two hours the downward cur- 

 vature of the free edge of the gills was marked and could be 

 detected with the naked eye. 



When the plane of a Mushroom pileus has not been tilted up 

 to an angle of more than about 30, all the gills can adjust them- 

 selves again so as to take up vertical positions. This is permitted 

 by the provision of a sufficient margin of safety in their spacial 

 arrangement. 1 When, however, the tilt exceeds a certain amount, 

 i Cf. Chap. II. 



