124 RESEARCHES ON FUNGI 



stronger. This is highly advantageous to the fruit-body as a whole, 

 as I shall now endeavour to show. One must bear in mind : (1) the 

 general mechanical principle that the more upright a fruit-body, 

 the less the moment of force acting about its base and tending to 

 break the stipe or overturn the fruit-body as a whole ; and (2) the 

 fact that the fruit-bodies of Coprinus sterquilinus are of consider- 

 able size and weight. The height of C. sterquilinus fruit-bodies 

 is often 9-12 inches and the width of the fully expanded pilei 

 3-6 inches ; and, for one large fruit-body, the weight was found to 

 be 27 grams and the centre of gravity 5 • 5 inches above the base of 

 the stipe. Since the weight of the pileus is considerable and the 

 stipe long, thin, and hollow, it is necessary for the mechanical 

 stability of a fruit-body as a whole that the stipe shall be upright 

 or almost upright. In open fields, where strong diffused light plays 

 upon a dung-mass from all sides, the stipes usually grow up in an 

 almost precisely vertical direction. If, however, a fruit-body grows 

 upwards in such a position that one side of it is close to a dark 

 obstacle (part of a dung-mass, etc.) and the other side looks toward 

 a well-lighted open space, then the slight heUotropic reaction that 

 takes place enables the stipe to slope very gently away from the 

 obstacle, making with the vertical an angle of about 10°, with the 

 result that, when the pileus comes to expand, the expansion is 

 unhindered by the obstacle ; and this, in the end, is distinctly 

 advantageous for the liberation of the spores. Since the slope from 

 the vertical is only about 10°, the fruit-body can still maintain its 

 fixed position without too much danger of falling. Were the angle 

 of slope to be increased to say 45° or 60°, the moment of force 

 acting about the base of the stipe would undoubtedly cause the 

 fruit-body to fall down or break. In the laboratory this was often 

 found to happen when, by slowly tilting the culture dish, the direction 

 of the axis of the stipe of an expanded fruit-body was changed 

 from the vertical or nearly vertical to the extent just indicated. 



In Volume I of these Researches, I stated that the stipes of 

 Coprinus comatus appear to be without response to the heliotropic 

 stimulus of light. 1 Since it has now been shown that the stipes of 

 Coprinus sterquilinus are shghtly positively heliotropic, I suspect 



^ These Researches, vol. i, 1909, p. 76. 



