PULSATING VACUOLES 299 



Sudden changes presumably exercise the customary shock-effect upon pulsation, 

 and it is possibly owing to some such action that only a few observers have been 

 able to detect an increased frequency when oxygen is deficient or carbon dioxide 

 abundant \ 



Functional importance. It is generally assumed that the contractile vacuoles aid 

 in the absorption of oxygen and other food-materials as well as in the excretion of 

 carbon dioxide and other waste products 2 . Maupas 3 has indeed calculated that 

 Infusoria may expel and reabsorb their own volume of water in two to forty-six 

 minutes. All Protozoa do not, however, possess pulsating vacuoles, nor do the cells 

 of Fungi or of primary meristems which also possess very active powers of respiration 

 and of metabolism. In tissues, however, the transit between the cell and the external 

 world becomes of greater importance and is slower than the entry into or escape from 

 particular cells. Hence the latter do not require special aids to absorption and 

 excretion, and in fact the excretion of water in the tissues of aerial organs, as occurs 

 when the filaments of Cynareae and irritable pulvini are stimulated, always involves 

 a certain hindrance to gaseous exchange external to the cell. It is possible also that 

 the contractile vacuoles may in some cases serve special purposes, and in the case of 

 Chilodon propellens each ejection of water causes a jerky movement of the organism in 

 the opposite direction ". 



SECTION 64. Other Protoplasmic Movements. 



All these movements, including those involved in cell and nuclear 

 division, are the direct or indirect results of vital activity, although their 

 detailed origin is comparatively unknown. All parts possessing the 

 properties of a viscous liquid must be subject to the physical laws 

 already discussed which determine or modify their shape. The flattened 

 character of many nuclei is, for instance, probably often due to the existence 

 of lateral pressure upon it. Every active enlargement of the nucleus, as 

 well as the growth of starch-grains, necessarily produce corresponding 

 displacements in the protoplasm, and in fact the expansion and contraction 

 of vacuoles may originate definite streaming movements in the protoplasm. 



The rounding of the viscous protoplasm on plasmolysis is the direct 

 result of the existence of a uniform surface-tension pressure at its external 

 boundary, but a bulging will always be produced at any point where 

 a lower surface-tension is maintained. An accumulation of the denser 

 constituents at one end of a cell produced by centrifugal action may, 

 however, take more than a week to be readjusted, whereas a rapid 



1 Biitschli, Protozoen, 1880-8, p. 1452. 



2 Cohn, Beitrage z. Biologic, 1877, Bd. ir, p. 118. For details see Btitschli, I.e., p. 1452. 



3 Quoted by Biitschli, I.e., p. 1455. According to Rhumbler (Archiv f. Entwickelungs- 

 mechanik, 1898, Bd. vn, p. 257), Amoeba proteus shows a pronounced contraction with every collapse 

 of the vacuole. 



* Engelmann, Zur Physiologic d. contractilen Vacuolen der Infusionsthiere, 1878. 



