Lentinus lepideus , Fr., to External Stimuli . 429 
attained a length of nearly seven inches in nine weeks (Figs. 19 and 25). 
It was still in the form of a stiff tapering rod, the diameter at the base 
being about three-eighths of an inch. At this stage it was changing from 
white to brown. 
The formation of the pileus depends entirely on the light. This is not 
the case for all Agaricini. Thus, as is well known, the pileus of the 
common Mushroom develops in a perfectly normal manner in completely 
darkened cellars. 
A very similar case to Lentinus lepideus is that of Coprinus stercorarius , 
investigated by Brefeld l . The stipe of this Fungus was observed to grow 
in the dark to the extraordinary length of between two and three feet. 
A tiny rudimentary pileus, never producing spores, was, however, always 
formed at the tip of the stipe. Lentinus lepideus in never showing the 
slightest trace of a pileus in the dark is, therefore, even more dependent on 
the light for the formation of that structure than the Coprinus. 
Some rotting blocks were placed in a damp-chamber in weak unilateral 
light, which was directed obliquely downwards at an angle of 45 0 with the 
vertical. Under these conditions the conical protuberances and rods were 
positively heliotropic. In whatever position the protuberances began their 
growth upon the blocks they gradually changed their direction of growth so 
as to bring their axes parallel to the incident rays. Further evidence of 
the positive reaction to heliotropic stimuli is afforded by the following 
experiments. 
A rod grew out from a block (Bl in Fig. 23), and placed its axis in the 
direction of weak rays of light indicated by the arrow a. When it had 
attained the length //, the block was moved round in a horizontal plane 
through two right angles so that the direction of the light was changed to 
that indicated by the arrow b . The rod grew in a curve in such a manner 
as to bring its axis once more parallel to the incident rays. A similar 
experiment was performed with the rods in Fig. 22. The rods 0, p , r, and s 
grew towards the light, the direction of which is shown by the arrow a. 
After the block had been turned round so that the direction of the light 
came to be that indicated by the arrow b , the rods 0 and p curved through 
a right angle towards the light. The rods r and ^ had stopped growing and 
therefore retained their original direction of growth. The rods t , v> w, &c., 
developed after the light had been changed to the direction indicated by the 
arrow b. 
Under well-lighted conditions the rods exhibit positive heliotropism, 
and usually, when they have attained the length of a few centimetres, 
proceed to the formation of a pileus. The pileus always arises terminally 
on the end of the rod which becomes the stipe (Figs. 10 and 14). The end 
1 Loc. cit., p. 90. 
