=31. Turgescence.=—Were it not for the resistance which the cell wall offers to the pressure from within, the delicate protoplasmic membrane would stretch to such an extent that it would be ruptured, and the protoplasm therefore would be killed. If we examine the cells at the ends of the threads of spirogyra we shall see in most cases that the cell wall at the free end is arched outward. This is brought about by the pressure from within upon the protoplasmic membrane which itself presses against the cell wall, and causes it to arch outward. This is beautifully shown in the case of threads which are recently broken. The cell wall is therefore _elastic_; it yields to a certain extent to the pressure from within, but a point is soon reached beyond which it will not stretch, and an equilibrium then exists between the pressure from within on the protoplasmic membrane, and the pressure from without by the elastic cell wall. This state of equilibrium in a cell is _turgescence_, or such a cell is said to be _turgescent_, or _turgid_. [Illustration: Fig. 14. Before treatment with salt solution.] [Illustration: Fig. 15. After treatment with salt solution.] [Illustration: Fig. 16. From salt solution placed in water. Figs. 14-16.—Osmosis in threads of mucor.] =32. Experiment with beet in salt and sugar solutions.=—We may now test the effect of a five per cent salt solution on a portion of the tissues of a beet or carrot. Let us cut several slices of equal size and about 5mm in thickness. Immerse a few slices in water, a few in a five per cent salt solution and a few in a strong sugar solution. It should be first noted that all the slices are quite rigid when an attempt is made to bend them between the fingers. In the course of one or two hours or less, if we examine the slices we shall find that those in water remain, as at first, quite rigid, while those in the salt and sugar solutions are more or less flaccid or limp, and readily bend by pressure between the fingers, the specimens in the salt solution, perhaps, being more flaccid than those in the sugar solution. The salt solution, we judge after our experiment with spirogyra, withdraws some of the water from the cell-sap, the cells thus losing their turgidity and the tissues becoming limp or flaccid from the loss of water. [Illustration: Fig. 17. Before treatment with salt solution.] [Illustration: Fig. 18. After treatment with salt solution.] [Illustration: Fig. 19. From salt solution into water again. Figs. 17-19.—Osmosis in cells of Indian corn.] [Illustration: Fig. 20. Rigid condition of fresh beet section.] [Illustration: Fig. 21. Limp condition after lying in salt solution.] [Illustration: Fig. 22. Rigid again after lying again in water. Figs. 20-22.—Turgor and osmosis in slices of beet.]