THE LEAF 41 
The above experiment can be performed very easily with a 
thistle tube and a piece of parchment paper, which is a semi- 
permeable membrane (Fig. 82). The paper is tied tightly over 
the large mouth of the bulb, the tube is inverted, and the bulb 
is filled with a sugar solution. The tube is now set in a dish of 
water with the tube end uppermost and the bulb about three- 
fourths immersed in the water. The sugar solution will absorb 
water and gradually rise in the tube. 
Osmosis in plant cells. The outer layer of the protoplasm of 
a plant cell is somewhat modified and is known as the plasma 
membrane. This acts as a semipermeable 
membrane around the cell, while the pro- 
toplasm as a whole is a semipermeable 
membrane around the vacuole. These 
membranes are of such a nature that water 
passes through them readily, while some 
of the substances which are in solution 
pass less readily and some not at all. The 
solution in living cells is highly concen- 
trated and is connected by means of the 
conducting tubes in the xylem with the 
roots, and through these with the water p,¢.33. A plasmolyzed 
in the soil; this water is a dilute solution ell from a hair of a 
of mineral salts. By the forces of osmo- squash shoot. (x 160) 
sis and hydration of colloids this water 
is drawn into the plant cells until the cell walls become stretched 
by the water. Cells that are thus stretched are said to be turgid, 
and the force of the stretching is called twrgidity. This turgidity 
gives considerable strength to the cells, as explained on page 34, 
just as a soft hose becomes hard when filled with water, or a 
bicycle tire extremely rigid when filled with air under pressure. 
If a cell is placed in a concentrated solution of sugar or of a 
~ mineral salt, this solution will pass through the cell wall and will 
draw water from the protoplasm. The removal of the water 
decreases the size of the cell contents so that the protoplasm is 
drawn away from the cell wall, as shown in Fig. 33. A cell in 

