Lentinus lepideus, Fr ., to External Stimuli . 431 
to the stimulus of light, but is a negatively geotropic reaction which can 
take place in complete darkness. As further evidence that the erection of 
the pileus is a geotropic and not a heliotropic phenomenon, it may be men- 
tioned that, when grown in the light, the pileus becomes erect however the 
light may be directed towards the fruit-body. 
The variations in shape of the mature pileus now require discussion. 
It was found that, when a stipe, before the formation of the pileus, was 
placed in a vertical position and rotated about its axis by means of 
a klinostat revolving horizontally under good light conditions, a symmetrical 
pileus with equal gills was formed. On the other hand, it was noticed that, 
when a fruit-body grew in an ordinary damp-chamber, it always had an 
oblique stipe (e.g. in Fig. 1), owing to the heliotropic reaction already 
explained. The pileus was then either symmetrically developed (Figs. 1 
and 11) or sometimes its development was very unequal (Figs. 9, 16, and 
26). In the latter case the longest gills were always on the lower side of the 
stipe, whilst the gills on the upper side were considerably shorter, or even 
undeveloped beyond the most rudimentary stage (Fig. 2 6). 
These variations appear to depend on the manner in which the fruit- 
bodies respond to the stimulus of gravity during the development of the 
pileus. The pileus in the youngest stage, as already stated, is symmetrically 
developed on the end of the stipe (Figs. 10 and 14). Let us now suppose 
that up to this stage in response to heliotropic stimuli the stipe has grown 
obliquely towards the light (Figs. 1 and 11). The end of the stipe below the 
decurrent gills then becomes negatively geotropic and bends upwards, thus 
bringing the pileus into the vertical position. Then, apparently according 
to the individual peculiarity of the fruit-bodies, either every side of the 
pileus elongates equally, and all the gills attain to the same length (Figs. 1 
and ij), or the side of the pileus which was on the lower side of the stipe 
grows much faster than the opposite side. The top of the pileus thereby 
finally becomes oblique. The gills on the side of the pileus where rapid 
growth has taken place become very much elongated, whilst those on the 
opposite side elongate but little or not at all (Figs. 1 6 and 2 6). 
The fact that when the gills are unequally developed, the longest ones 
are always on the lower side of the stipe, suggests that the inequality 
is brought about by the stimulus of gravity. The possible action of light 
must, however, be taken into account, for, owing to the heliotropic reaction 
of the young stipe, it so happened that the best developed side of the pileus 
was not only on the lower side of the stipe, but also received most light. 
A young, obliquely-growing fruit-body, upon which a pileus was just begin- 
ning to form, was, therefore, taken and placed in such a position that the 
light was directed towards the upper side of the stipe and pileus. The gills 
developed as before, most vigorously on that side of the pileus which was 
continuous with the lower side of the stipe. The least lighted side of the 
