THE MATERIAL INCOME OF PLANTS 309 
The condition of cell walls in a massive tissue may be comprehended clearly by 
inspecting a mass of bubbles such as may be formed by blowing air through a tube 
into a soap solution. 1 The outer bubbles will have a convex surface, but plane 
films divide the air bubbles in the interior. Pricking a superficial bubble gives 
opportunity for the plane walls of those adjacent to it to bulge, because the in- 
ternal pressure is now unbalanced. 
A cell thus overfilled with water, with the elastic wall stretched, or 
under strain and ready to stretch, is said to be turgid, and the condition 
is designated as turgidity. Manifestly, turgidity depends upon two fac- 
tors : the presence of a solute or solutes in sufficient amounts, and an 
adequate supply of water. 
Turgor and osmotic pressure. The pressure developed within the 
cells, when an adequate amount of water is at hand, may equal the os- 
motic pressure of the solutes to which the cytoplasm is impermeable. 
Obviously, the osmotic pressure exists, whether or not it exhibits itself; 
it exhibits itself by stretching the elastic container only when sufficient 
water can enter; this particular exhibition of it is known as turgor? or 
turgor pressure. Thus within the cell there exists both osmotic pressure 
and turgor pressure; the latter is a sort of hydrostatic pressure depend- 
ent upon the former for its existence and probably upon the resistance 
of the protoplast and the cell wall to filtration for its amount. It is 
seldom likely, therefore, to equal the osmotic pressure. 
Thus, in the cells of the sugar beet, the cane sugar alone has an osmotic pressure 
of 10 or ii atmospheres; and there are certainly many other solutes which would 
add greatly to this. But the turgor pressure can only reach a point at which water 
will filter through the cytoplasm and cell wall, and this is probably less than half the 
osmotic pressure of the sugar alone. 8 
That the osmotic pressure is always ready to produce turgor is shown 
by the fact that flaccid cells placed in pure water quickly become 
turgid. 
Plasmolysis. If a turgid cell is placed in a solution more concentrated 
than that within it, water emigrates from the cell, which then becomes 
more or less flaccid. By measuring turgid cells, or making careful 
1 This may be made of a plain glycerin soap. More durable bubbles may be made 
from this solution : Shaved white Castile soap 10 gm. by weight; warm water 400 cc.; 
dissolve. To 1 5 parts by volume, add glycerin 1 1 parts. This will be improved by al- 
lowing it to stand for a week, cooling over night to 3 C. and filtering cold until limpid. 
2 The term is not always thus restricted; it is often used as synonymous with turgidity. 
3 Further studies of this subject are much needed, especially as the usual mode of test* 
ing osmotic pressure by plasmolysis has been shown to be faulty. 
