158 Br. J. Burdon-Sanderson [June 9 



And now let me pass to another group of plants which may serve 

 as a contrast to Stylidium. Stylidium may be called an out-of-the- 

 way plant. It has an organisation which is not represented in the 

 European flora. The family of thistles, and their allies the knap- 

 weeds (represented in our gardens by the ladies' blue-bottle), all of 

 which are common wayside plants, exhibit excitable movements 

 which, although of a very different kind from those we have just 

 described, have, like them, to do with the visits of insects for the 

 purpose of fertilisation. We will now throw on the screen a single 

 fertile floret of Centaurea Cyanus (Fig. 5). The large diagram shows 

 the same floret deju'ived of its corolla. Its axis is occupied by the 

 style, surrounded by its tube of anthers. Below, the anther-filaments 

 expand into a kind of cage, and again a2:)proach one another, when 

 they are united with the tube of the corolla. At the moment that 

 the anthers arrive at maturity these filaments are very excitable. 

 When one of them is touched, it contracts and draws the style towards 

 itself. Immediately afterwards the excitatory eftect spreads to the 

 others, all five arches becoming straight and api)lying themselves 

 closely to the style. A similar effect is produced by an induction 

 shock. [The structure described was projected on the screen; on 

 passing an induction current through it, the mode of contraction of 

 the filaments was seen.] 



The mechanism of Centaurea has been studied by many plant 

 physiologists, particularly by Professor Ferdinand Cohn of Breslau, 

 and more recently with great completeness by Professor Pfeffer. It 

 has in this respect a greater interest than any other — that the shorten- 

 ing of these filaments in response to excitation strikingly resembles 

 muscular contraction. You have here a structure in the form of a 

 flattened cylinder which resembles many muscles in form, the length 

 of which is diminished by about a sixth on excitation. This super- 

 ficial resemblance between the two actions makes it the more easy to 

 appreciate the differences. 



Let me draw your attention to the diagi-am of an experiment 

 made last year, which was intended to illustrate the nature of muscular 

 contraction, and particularly to show that when a muscle contracts, it 

 does not diminish in volume. The first difference between muscle 

 and plant is a difference in the degree of shortening. A muscle 

 shortens by something like a third of its length, the anther filament 

 only by a sixth. But it is much more important to notice that in 

 contracting, the filaments do not retain their volume. In shortening, 

 they broaden, but the broadening is scarcely measurable ; hence they 

 must necessarily diminish in bulk, and this shrinkage takes place, as 

 Pfeffer has shown, exactly in the same manner as that in which the 

 excitable cushion of Mimosa shrinks, namely, by the discharge of 

 liquid from its cells. 



We are now in a position to study more closely the question to 

 which I referred a few minutes ago — How do the cells discharge their 

 contents ? The structure of the filament of Centaurea, from its 



