358 VEGETABLE PHYSIOLOGY 



peripheral portions of the protoplasm. These two cases of 

 streaming movements of protoplasm are spoken of as rotation 

 and circulation respectively. There is no difference appa- 

 rently 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 Chara and 

 Nitella, and the unicellular Desmids. 



It is evident from the structure of most vegetable organisms 

 that the possession of a power of active contractility, 

 such as is possessed by most animals, would be of com- 

 paratively 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 proto- 

 plasm and water. Each cell or protoplast is so organised 

 as to contain its own appropriate store, upon the posses- 

 sion and renewal of which its efficiency as a member of 

 the colony, if not its actual life, depends. The regulation 

 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 overfull 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 possesses 

 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 cell, as we 



