APPENDIX TO CHAPTER II. 
Turgor of the Cell.—The cytoplasm is a semi-permeable mem- 
brane; that is, it exercises control over the passage of the cell- 
sap, containing salts in solution, outwards, though it allows the 
passage of water inwards. As a consequence the cell continues 
to absorb water until the cell-wall is tightly stretched. A cell 
in this condition is said to be turgid or in a state of turgor. If 
such a living turgid cell is treated with a 10 per cent. salt solu- 
tion, it will be found that the following changes occur. The cell 
first decreases in size. The cytoplasm contracts away from the 
cell-wall ahd rounds itself off into a spherical or oval mass in the 
middle of the cell, This phenomenon is known as plasmolysis. 
The crispness and firmness of herbaceous plants depend on 
the turgidity of the cells. So long as the cells are turgid, so 
long will the tissues be firm. But if there is a deficiency in the 
water supply, so that the cells are no longer turgid, then the 
tissues become limp and the plant fades. A renewal of the 
water supply will make the cells turgid again, unless they are 
dead. It is to be noted that this control over the cell-sap is 
essentially a property of living protoplasm ; when the cells are 
killed, the control is lost, and the sap will diffuse out of the cells. 
Experimental Work.—To observe plasmolysis, scrape some 
hairs off the stem of a cucumber, or some other hairy plant, 
mount them in water on a slide, and examine under the high 
power of a microscope. Make out the granular cytoplasm, the 
nucleus, and the vacuole filled with cell-sap. Irrigate with a 
10 per cent. sali solution and watch the cells. (The irrigation is 
best done by putting a few drops of salt solution on one side of 
the cover slip and a piece of blotting-paper on the other. The 
blotting-paper sucks the solution through.) After the cells have 
been plasmolysed, they can be irrigated with fresh water until 
they are turgid again. For comparison cut some thin sections 
of fresh beetroot and treat them in the same way. . 
Cut some slices of beetroot (fresh and unboiled), measuring 
about 2 inches by 1 inch by linch. Immerse some of these in 
a beaker of fresh water, and others in a 10 per cent. salt solu- 
tion. After a few hours take out both sets of slices. Note that 
those in the fresh water are hard and stiff, whereas those in the 
salt water are limp and flaccid. If these latter,are transferred 
to fresh water they will soon become firm again. 
Take a small test tube, fill it with a 10 per cent. salt solution 
and tie a piece of bladder tightly over the top. Now immerse 
the tube in fresh water and leave for an hour or more. Note 
that the bladder is now swollen out; that is we have made an 
imitation of a turgid cell. If this imitation cell is now immersed 
in a 20 per cent. salt solution it will be seen that the bladder 
becomes concave instead of convex, ie. the cell is no longer 
turgid, 
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