198 
PROGRESS OF MICROSCOPICAL SCIENCE. 
based on the distribution of the cranial nerves, would have in futuro 
to take the olfactory nerves into consideration. 
Protoplasmic Filaments of the Teasel. — Mr. F. Darwin, M.B., bas 
read a valuable paper before the Royal Society,* on this subject. The 
structures described are, he says, connected with the glandular hairs 
or trichomes found on both surfaces of the leaf of the common teasel 
( Dipsacus sylvestris). The trichomes are of two kinds, differing in a 
marked manner in shape. The gland consists of a multicellular pear- 
shaped head, supported on a cylindrical unicellular stalk which rests 
on a projecting epidermic cell. The whole structure projects about 
-jijj of a millimeter inch) above the surface of the leaf. 
The filaments issue from inside the gland-cells, reaching the sur- 
rounding medium by passing through the external cell-wall of the 
gland. The point where protrusion takes place is on the summit of 
the gland, and usually at the point of junction of several radiating 
cells at the centre of its dome-like surface. The act of protrusion is 
rapidly effected ; a previously naked gland may be seen to send forth 
a minute thread of trembling protoplasm, projecting from its summit 
freely into the surrounding water. The filament grows by clearly 
visible increments, and may ultimately attain the length of nearly one 
millimeter. The filaments appear to pass throughout the substance 
of the external cell-wall of the glands, as no apertures to allow of 
their passage have been observed. 
Under normal circumstances the filament presents the appearance 
of a delicate and elongated thread slightly clubbed at its free end, and 
animated by the perpetual tremble of Brownian movement. The 
distal end of the filament is often attached to the gland, thus forming 
a loop. Extremely delicate filaments of great length are often seen 
entangled in elaborate and complex knots, or several filaments may be 
seen issuing from a single gland. The substance of which the fila- 
ments are composed is gelatinous, transparent, highly refracting, and 
devoid of granules. It is in a great measure soluble in alcohol, is 
stained by tincture of alkanet, and not blackened by osmic acid, and 
is coloured yellow by iodine. These reactions, when combined with 
results of various physiological tests, show that the filaments contain 
resinous matter in some way suspended in protoplasm. 
The most remarkable point in the behaviour of the filaments is 
their power of violently contracting. The act of contraction com- 
mences by the filament becoming shorter and thicker at a number of 
nearly equidistant points, situated close together near the free end of 
the filament. The curious beading thus produced spreads rapidly 
down the filament, which ultimately runs violently together into a ball 
seated on the top of the gland. In other cases contraction takes place 
without any previous appearance of beading. 
Filaments frequently break loose but retain their vitality, and are 
still capable of contraction although separated from their parent 
glands ; and this observation is of importance, as proving that the 
movements of the filaments are not governed by forces residing within 
* ‘ Proceedings of the Roy. Soc.,’ No. 179. 
