Lentinus lepideus , Fr to External Stimuli . 435 
fruit-bodies of Lentinus lepideus are extraordinarily variable. We have 
seen that under some conditions they are practically without a stipe ; that 
under others they possess stipes three inches long ; that sometimes the 
pileus is excentrically developed ; that in the dark the fruit-bodies assume 
the form of tapering rods, attaining a length of some six inches but with no 
trace of a pileus, and that finally in weak light they often develop into 
curious branched monstrosities (cf. Figs. 15, 11, 16, 26, 25 and 28). 
Agaricus campestris in comparison with Lentinus lepideus is a very 
stable form. Its fruit-bodies grow quite normally both in sunlit fields and 
in completely darkened cellars. They always develop a stipe and a sym- 
metrical pileus bearing gills, which produce spores. The stipe and pileus 
are never branched. One may inquire why the one Agaric should be 
so variable, and the other comparatively stable. I will venture the sugges- 
tion that the difference is connected with the habitat of the two Fungi. 
Agaricus campestris is a ground Fungus. The orientation of its substratum 
is in so far definite that the surface is on the average horizontal. This 
being so, when a fruit-body has once begun to form on the surface of the 
ground, suitable reactions to the stimulus of gravity alone are necessary to 
bring about the development of the stipe and pileus in vertical sequence, 
and the subsequent uplifting of the latter into the air. Hence we find that 
the development of the Mushroom is controlled by the stimulus of gravity 
and is practically unaffected by light. On the other hand, Lentinus lepideus 
is a tree Fungus. The orientation of the surface of its substratum is 
indefinite. It may be in any position whatsoever with regard to the fruit- 
bodies. In order that the latter may be brought into the open air, they are 
provided with the power of reacting to the stimulus of light, the advantage 
of which has already been explained (p. 433), as well as to the stimulus 
of gravity. The adaptation of the two Fungi to different conditions of life 
has thus led to the acquirement of different physiological properties, with 
the result that Lentinus lepideus has become physiologically much more 
complex than Agaricus campestris , and, therefore, much more liable to 
variation. If the foregoing remarks upon the physiological differences 
between the two Fungi hold good, further investigation should show that 
in general tree-Agarics (and possibly also dung- Agarics) behave to light 
and gravity like Lentinus lepideus , whilst ground-Agarics behave like 
Agaricus campestris . 
Summary. 
The fruit-bodies of Lentinus lepideus were grown upon rotten paving 
blocks, taken from the streets of Birmingham. The Fungus belongs to 
the Agaricini. 
The papillae, from which the fruit-bodies arise, are not somatotropic, 
so far as the wooden substratum is concerned, but grow out perpendicularly 
