62 i{EsEAKCJii:s OX Frxca 



When the fniit-body hccoraes 1'u11-il,ti)\v)i, it thus comes to have 

 an iiiiiIm'cU:! shape hke that of an u]trit(ht, expanded Mushroom 

 (Fii^-. 22). On the other hand, if the axis of tlie conical process 

 upon the stromatous knoh happens to he incHncd upwards, let 

 us suppose at an angle of 4")°, then the fruit-l)ody at its origin 

 is oliliipiely set (Fig. 21, B: Plate V., Fig. ;'2). The young sti|)e 

 receives a directive stimulus from gravity and grows faster below 

 than above. It therefore gradually bends upwards and only stops 

 this movement when the top of the pilous has been turned into 

 a horizontal position {cf. Fig. 21, D). We may suppose that a 

 morphogenic stimulus of a special kind is conducted to the })ileus 

 from the stipe whilst this is making its geotropic curvature. As 

 a reaction to this stimulus the pileus undergoes a physiological 

 change : its developmental forces become reorganised. It can 

 now be said to have two distinct halves differentiated from one 

 another. One of them may be referred to as anterior in position 

 and the other as posterior. The former is always the h:df which 

 is furthest away from the base of the stipe and the latter the one 

 nearest to it (Fig. 21, F-I). The anterior half undergoes rapid 

 and considerable expansion, but the posterior half soon discontinues 

 its development. In the mature fruit-body, in this instance, the 

 pileus has a unilateral position like that shown in Fig. 4, p. 28, and 

 in Fig. 21, E. Slightly eccentric pilei are formed when their stipes 

 at the beginning happen to be nearly vertical but not quite. The 

 less oblique the stipe, the less will be the physiological diU'er- 

 entiation induced by the stipe in the two halves of the pileus, 

 and the more nearly in the end will the fruit-body approach the 

 umbrella form. If the foregoing correctly represents what takes 

 place during development, it follows that the {<^^x\\\ of a I'ruit- 

 body is indirectly controlled by gravity. The radial .symmetry 

 of the pileus is only interfered with when the stipe has responded 

 to a geotropic stimulus. The degree of eccentricity of the one 

 part seems to be proportional to the amount of curvature luidergone 

 by the other part. 



Since the unilateral pilei of fruit-bodies growing on the trunks of 

 trees are directed away Irojii the trunks ov ln-aiiches, one might be 

 inclined, withont investigation, to think that this is due in part to 



