6^4 LECTURE XXXVII. 



ordinary non-irritable cells, where the previously slightly extended cellulose wall 

 contracts, but not to the same degree, so that between it and the contracted proto. 

 plasmic utricle there arises an interspace filled with water, as in Fig. 189. If we 

 now suppose that the alteration of the protoplasmic utricle just mentioned actually 

 occurs, in consequence of a touch, shake, sudden illumination, electrical disturbance, 

 or any other stimulus, but that the corresponding contraction of the rigid cell-walls 

 does not occur, then no movement of the organ would be perceived externally, 

 although the protoplasmic utricles had been irritated and had, in fact, reacted to 

 the stimulus. We see thence that in the case of the irritable organs of plants two 

 essentially distinct points come into' consideration ; on the one hand the action of the 

 stimulus on the protoplasm, and on the other the extensibility and elasticity of the 

 cellulose wall. Nature herself presents us with examples of the case here only 

 assumed for purposes of illustration. In Spirogyra (Fig. 2 23, p. 3 14) the protoplasmic 

 utricle which was hitherto closely applied to the cell-wall contracts, for the purpose 

 of subsequent conjugation, into a rounded vesicle, and this is evidently only possible 

 by the water of the cell-sap escaping out through its substance. The protoplasm, 

 however, separates from the cell- wall in the process, as in a plasmolysed paren- 

 chyma-cell (Fig. 189), because the cell-wall itself was only slightly extended pre- 

 viously, and accordingly only suffered a slight contraction. If the cellulose wall of 

 such a Spirogyra cell had been previously much distended, and if then the proto- 

 plasmic utricle became contracted, water being expelled, the cellulose wall would 

 also contract and the entire cell would act like an irritable organ. Evidently we 

 might suppose the tissue of an irritable organ of Mimosa, or of a stamen of 

 Cynara, composed of cells resembling those in Spirogyra, arid then the conse- 

 quence of a stimulus would be that their protoplasmic utricles would contract, but 

 the cellulose walls would remain unaltered, and we should perceive no external 

 movement. Indeed it is not at all improbable, that such processes actually occur 

 in the ordinary apparently non-irritable parenchyma of plants, since we know, 

 as a matter of fact, that in many Algae, hairs, &c., the protoplasm becomes con- 

 tracted by mere pressure from without, the cell-wall not contracting to the same 

 extent. 



In conclusion I will mention, simply in passing, the irritable stamens of the 

 genus Berberis. These, six in number, stand in a circle around the central ovary, 

 and in the opened flowers are thrown radially outwards. A slight touch on the inner 

 side causes the filament to dart suddenly inwards, so that the anther comes to lie on 

 the stigma. If an electric current, generated by numerous small elements, is con- 

 ducted through the flower in such a way that it runs longitudinally through the 

 pedicel and ovary to the stigma, or takes the reverse direction, the following remark- 

 able phenomenon is observed: each time the current runs from the stigpaa to the 

 pedicel a stimulation of all the stamens follows, while a current in the reverse 

 direction causes no stimulation. I discovered this fact so long ago as 1878, but have 

 not had an opportunity since of pursuing it further; it would certainly be not 

 uninteresting to know, whether a similar phenomenon occurs in other irritable organs 

 also. 



The irritable stamens oi Berberis differ, as to their mechanism from those of the 

 Cynarese considerably, above all in that they are irritable only on the inside and not on 



