350 VEGETABLE PHYSIOLOGY 



streaming movements of protoplasm are spoken of as 

 rotation and circulation respectively. There is no differ- 

 ence apparently between them, except what is involved in 

 the different distribution of the protoplasm in the cells. 

 Other instances are met with in the staminal hairs of 

 Tradescantia (fig. 150), the leaves of Vallisneria, the 

 internodal cells of Char a and Nitella, and the unicellular 

 Desmids. 



It is evident from the structure of most vegetable 

 organisms that the possession of a power of active con- 

 tractility, such as is possessed by most animals, would be 

 of comparatively little use to them. Though flexible to a 

 certain extent, they are possessed of a fair amount of 

 rigidity, which under ordinary conditions they do not 

 relax. We have seen that one of the most important 

 relations of their life is that which is maintained between 

 the protoplasm and water. Each cell or protoplast is so 

 organised as to contain its own appropriate store, upon the 

 possession and renewal of which its efficiency as a member 

 of the colony, if not its actual life, depends. The regula- 

 tion of this supply of water is of the first importance to the 

 plant, and it is not surprising, therefore, to find that such 

 a regulatory power is one of the properties of vegetable 

 protoplasm. 



All healthy vegetable cells are during life in a condi- 

 tion which is known as turgor. The cell is overful of 

 water, so that a certain internal hydrostatic pressure is 

 exerted on the whole surface of the limiting membrane, 

 which is stretched accordingly. As the membrane pos- 

 sesses elasticity, the wall in turn exerts a pressure upon 

 the fluid inside it, and during healthy life a certain 

 equilibrium exists between these two pressures. Such a 

 cell is called turgid, and the degree of its distension is 

 the measure of its turgidity. This turgor can vary within 

 fairly wide limits, consistently with the health of the cell. 

 The turgor depends chiefly upon two factors, both of which 

 are capable of control. The water is caused to enter the 



