ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 639 



vacuoles. He demonstrated that the vacuoles are enclosed in a 

 definite membrane * by the application of a 10 per cent, solution of 

 potassium nitrate, which kills the rest of the protoplasm without 

 bringing about any change in the membrane of the vacuoles. The 

 " aggregated masses " are in reality vesicles filled by fluid contents, 

 each vesicle being a part of a vacuole with its enclosed cell-sap. The 

 irritation which excites Drosera and other insectivorous plants to an 

 increased development of their secretion, brings about peculiar and 

 very active movements in the cells of the tentacles, and of their stalks. 

 These movements consist chiefly of three factors, viz. : — (1) An in- 

 creased and much more strongly differentiated circulation of the 

 parietal protoplasm ; (2) A division of the vacuoles into a larger or 

 smaller number of portions, each of which is enclosed in a part 

 of the original membrane of the vacuole ; (3) A very considerable 

 diminution of the volume of these vacuoles, a portion of their mass 

 being expelled through the membrane and collecting between it and 

 the circulating protoplasm. This expelled fluid possesses, at least 

 approximately, the same attractive force for water as the rest, but a 

 different chemical composition, the pigment and certain dissolved 

 albuminous substances not being expelled along with it. These 

 albuminous substances can be separated by means of ammonia salts 

 in the form of a finely granular precipitate, which gradually collects 

 into larger balls, and is at first soft but afterwards harder ; but this 

 does not take place in the normal process of aggregation. When the 

 action of the irritant ceases, the cells gradually return to their 

 original condition, the vacuoles again increasing and coalescing. 



The observations were made chiefly on the marginal tentacles of 

 the leaves of Drosera rotundifolia ; also on D. intermedia and spathulata, 

 and on Pinguicula vulgaris. The substance used for exciting the 

 irritation was small particles of white of egg. 



Influence of Mechanical Forces on Cell-division, &c.f — Accord- 

 ing to researches by Herr E. Hoffmann, a unilateral strong positive 

 pressure on the cambium-cells may check or even prevent growth in 

 the direction opposed to the pressure. If the pressure acts obliquely 

 on the dividing cells the rows of cells deviate from their normal 

 position. When the bark-pressure has not only disappeared, but 

 become negative, as in depressions in the surface of the stem where 

 the tension of the bark is uniform, the cell-divisions appear to increase 

 in frequency, young stems becoming cylindrical instead of angular. 

 When a stem is wounded, the normal pressure of the bark is removed 

 from the cambium, causing a stronger growth of the stem in the 

 neighbourhood of the wound ; and the cells formed on the margins of 

 wounds are isodiametrical until the normal conditions of pressure are 

 restored. The cause of the lateral growth of cortical cells on a 

 wound is traced in the same way. 



* Cf. supra, p. 637. 



t Hoffmann, R., 'Unters. iiber die Wirfcung mechanischer Kr'afte auf die 

 Theilung u.s. w. der Zellen,' 24 pp. (4 pis.), Berlin, 1885. See Bot. Centralbl., 

 xxv. (1886) p. 359, 



