4 



Mr, F. Darwin on the Protrusion of 



[Mar. 1, 



traversed by the current. They show also that when these liquid non- 

 conductors are mixed with the feeble conductor alcohol, the conductivity 

 of the mixture is greater than that of alcohol alone, which offers at least 

 a partial clue to the readiness with which such mixtures are decomposed 

 by the copper-zinc couple. 



The very considerable development of heat in these liquids, which 

 conduct the electric current with great difficulty, is a circumstance 

 worthy of notice. In these cases it is evident that it does not result 

 from any chemical change, because the decomposition, if any thing at all, 

 is utterly insignificant in amount. 



II. " On the Protrusion of Protoplasmic Filaments from the Glan- 

 dular Hairs of the Common Teasel {Dipsacus sylvestris)." 

 By Francis Darwin,, M.B. Communicated by Charles 

 Darwin, F.R.S. Received January 30, 1877. 



(Abstract.) 



The protoplasmic structures described in the following communication 

 are connected with the glandular hairs or trichomes found on both sur- 

 faces of the leaf of the common teasel (Dipsacus sylvestris). The trichomes 

 are of two kinds, differing in a marked manner in shape. The form of 

 gland from which alone the protoplasmic filaments issue is shown in the 

 diagram. 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 T ^ of a millimetre ( ^ inch) above 

 the surface of the leaf. 



The filaments issue from inside the gland-cells, reaching the surrounding 

 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 sur- 

 rounding water. The filament grows by clearly visible increments, and 

 may ultimately attain the length of nearly one millimetre. The filaments 

 appear to pass through 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 ani- 

 mated by the perpetual tremble of Brownian movement. The distal end 

 of the filament is often attached to the gland, thus forming a loop. Ex- 

 tremely 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. 



