ADJUSTMENTS OF FRUIT-BODIES 49 



thus comes to have its axis turned upwards into a vertical 

 position. The gills at the beginning of their development simply 

 grow outwards in directions which are perpendicular to the under 

 surface of the pileus. Their only reaction to external stimuli 

 appears to be that of positive geotropism, which comes into play 

 as soon as they have attained a certain breadth. 



The turning movement which is necessary in order to bring 

 the planes of the gills into exactly vertical positions, in the main 

 is accomplished by the stipe, but for its completion the sensitive 

 gills are themselves alone responsible. In the work of securing 

 a proper orientation for the hymenial surfaces, the stipe acts as 

 a coarse adjustment and each gill as a fine adjustment. In 

 their nature and successive action these two adjustments are 

 strictly analogous to those which are employed in focussing the 

 high power of a microscope. 



The reactions to external stimuli which have just been detailed 

 are such that : — 



(1) The barren stipe grows in a manner suited to find a Avay 

 to the open air. 



(2) The pileus is never developed in any space which is shut 

 out from daylight, and therefore of such a character that, if spores 

 were liberated into it, they could not be properly disseminated, 



(3) As soon as a pileus has begun its development, its hymenium 

 can readily be placed in the optimum position by a suitable 

 curvature of the stipe combined with a subsequent adjustment 

 of the gills. 



The fruit-bodies of Lentinus lepideus, when growing out from 

 the side of a piece of wood, to some extent exhibit the phenomenon 

 of eccentricity of development. In extreme cases, under cultural 

 conditions, the pileus flesh may become quite unilateral (Fig. 16, E). 

 I have shown that this is due to a morphogenic stimulus of 

 gravity acting upon a pileus developing upon an oblique stipe. 

 There can be no doubt that the reaction is advantageous in that 

 it permits of the fruit-body developing the chief part of its spore- 

 producing surface in a situation where the spores will run the 

 least risk of catching upon the stipe whilst making their escape. 



^ Loc. cif., p. 431, 



D 



