432 Buller . — The Reactions of the Fruit-Bodies of 
pileus had here developed best. Another fruit-body grew out from near 
the middle of the vertical side of a paving block in a horizontal direction 
and almost parallel to the surface of the wood during rotation of the block 
upon a horizontally revolving klinostat. Under these conditions, whilst the 
pileus was turning upwards and becoming unequally developed, the side 
next to the black surface of the block received least light, that turned away 
from the block received most light, and the sides continuous with the upper 
and under sides of the stipe were fairly equally illuminated. Nevertheless, the 
least and best lighted sides of the pileus developed gills of equal length : the 
side of the pileus continuous with the lower side of the stipe developed 
the longest gills, and that continuous with the upper side the shortest, 
although these two sides had been about equally illuminated. Want of 
sufficient fruit-bodies at this stage of the work unfortunately prevented the 
carrying out of further experiments of a similar nature to the two described. 
The evidence obtained, however, is in favour of the view that when a pileus 
is developed unequally, the stimulus of gravity and not that of light induces 
the inequality. 
The young pileus is convex above (Fig. i). During further develop- 
ment the margins of the pileus turn upwards so that the top becomes 
depressed and concave (Fig. 5). During the change the gills undergo con- 
siderable elongation. Their outer margins become stretched to such an 
extent that they finally become very much torn and thus assume a serrated 
appearance, which is characteristic for the genus Lentinus 1 (Figs. 5, 8, 
and 11). 
As the gills develop they become strongly positively geotropic and 
can alter their direction of growth so as to bring themselves into vertical 
planes. A fruit-body, which had developed its gills in the ordinary 
manner, was turned through a right angle and placed in the dark (Fig. 7). 
In two days the gills had altered their direction of growth so that they were 
all directed downwards (Figs. 8 and 9). During an experiment on geo- 
tropism, already described, the pileus ceased to be moved round at an 
angle of 45 0 with the vertical. The gills under these conditions were 
no longer vertical with respect to the earth, but inclined to it. In about 
two days the gills altered their position so as to bring themselves into 
vertical planes as far as circumstances would allow. They all turned 
downwards towards the earth and thus exhibited positive geotropism 
(Fig. 5). When grown in unilateral light the gills show no reaction to light 
in the direction of their growth. They are, therefore, in no way heliotropic. 
The reactions of the fruit-bodies to the stimuli of light and gravity can, 
I think, be explained on oecological grounds without much difficulty. It 
should be remembered that, so to speak, the main object of a developing 
fruit-body is to form its pileus in the open air, so that the spores may 
1 G. Massee, British Fungus-Flora, vol. ii, p. 299. 
