Lentinus lepideus , Fr ., to External Stimuli . 433 
be freely disseminated by the wind. In the case of seedlings of green 
plants the young shoot, whilst still covered with soil, is almost universally 
negatively geotropic. This reaction is admirably adapted for bringing the 
shoot into the light, for the surface of the soil is on the average horizontal. 
The surface of the wooden substratum on which fruit-bodies of Lentinus 
lepideus develop (i. e. the surface of fallen logs, &c.) is not on the average 
horizontal. It is in any direction whatsoever. It would be of no advan- 
tage, therefore, to the young fruit-bodies to have a specific geotropic 
reaction. They are, therefore, ageotropic. It is, however, advantageous 
that the direction of growth should be controlled by light, and, accordingly, 
the young fruit-bodies are positively heliotropic. Very weak light will 
cause them to grow outwards from the substratum through cracks in bark, 
leaves, and other obstacles, so that in the end the growing points will reach 
the open air. Here stronger light will cause the pileus to develop. The 
dependence of the pileus for its development on light ensures that the 
structure shall only be produced in suitable places for the distribution 
of the spores. As soon as the pileus has begun its development in the 
open, the end of the stipe ceases to be heliotropic and becomes negatively 
geotropic. This change is of the highest advantage. Negative geotropism 
leads to the erection of the pileus, by which operation the best position and 
spacial arrangement is provided for the spore-bearing gills. Continued 
positive heliotropism would only interfere with the erection of the pileus. 
Hence, when the latter is maturing, the reaction becomes eliminated. 
If the axis of the pileus is erect, the gills, by simply growing outwards 
radially, are at once in vertical planes, i. e. in the best position for the falling 
of the spores. If, however, any obstacle should prevent the axis becoming 
erect, then the gills, owing to their positive geotropism, grow vertically 
downwards in order to place themselves, so far as the falling of the spores 
is concerned, in the best possible position under the circumstances. 
Summing up this discussion, the conclusion seems to be that the 
various reactions of the fruit-bodies to the stimuli of light and gravity 
are adaptations for ensuring that spore-production shall take place in 
the most economical manner. 
A copious excretion of somewhat viscid and often red drops frequently 
takes place from stipe, top of pileus or gills (Figs. 10 and 15), provided that 
the air of the damp-chamber is saturated with water-vapour. The excretion 
of the drops may be started or increased by watering the paving blocks, on 
which the fruit-bodies are growing. The excretion, therefore, probably 
serves the purpose of getting rid of excess water from the mycelium. 
The colourless spores are shed in vast numbers. Their fall appears, 
as probably for most Agaricini, to be largely determined by the hygro- 
scopic state of the atmosphere. Half of the pileus was cut away from 
a fruit-body and placed upon a piece of black paper in a dry situation. 
