94 LIVERPOOL BIOLOGICAL SOCIETY. 
angle from the body. If the contact be gentle the bulb is 
cracked off at a junction with the body, the sharp edge of 
the latter apparently causing the wound. If the contact 
be rough the hair is broken across the middle instead, and 
the wall in that part seems not to be sufficiently firm to 
cause any puncture in the skin. There flows out imme- 
diately a portion of the granular contents of the apex of 
the hair, which, from its being stainable by dyes and from 
its chemical reactions, appears to be as truly protoplasmic ~ 
as the contents of the hair generally. The contents of the 
vacuoles were not expelled in all the cases which we 
observed. We conclude from this that the irritant is to 
be sought for in the protoplasm itself. We hazard no 
suggestion as to its chemical nature, but reserve our 
conclusions for a further note, which we desire to present 
sO soon as our investigations are concluded. 
The mechanism of expulsion we judge to be as follows. 
The pedicel cells form a reserve of turgid parenchyma, 
from which the base of the stinging hair receives a supply 
by osmosis. Internal pressure in the latter is thus set up, 
which is relieved by the breaking off of the bulb head, 
some of the apical contents being at the same moment 
expelled by elasticity of the cell wall. 
EXPLANATION OF PLATE I. 
Fig. 1. Stinging hair of Urtica dioica, x 50. 
Fig. 2 and 8. Ordinary epidermal hairs, x 50. 
Fig. 4. Glandular hair, x 50. 
Fig. 5. Base of the stinging hair, x 350. 
Fig. 6. Apex of the stinging hair, x 500. 
