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 Vallisnerta, 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 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 
