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 Tradescantva (fig. 150), 
the leaves of Vallisneria, the internodal cells of Chara and 
Nutella, 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 maintammed 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 
