18 PHYSIOLOGY. 



ward through the protoplasmic membrane, so that the latter is pressed outward 

 until it comes in contact with the cell wall again, which by its elasticity soon 

 resists the pressure and the cells again become turgid. 



35. The coloring matter in the cell sap does not readily escape from the 

 living protoplasm of the beet. — The red coloring matter, as seen in the sec- 

 tion under the microscope, does not escape from the cell-sap through the pro- 

 toplasmic membrane . When the slices are placed in water, the water is not 

 colored thereby. The same is true when the slices are placed in the salt or 

 sugar solutions. Although water is withdrawn from the cell-sap, this coloring 

 substance does not escape, or if it does it escapes slowly and after a consider- 

 able time. 



36. The coloring matter escapes from dead protoplasm. — If, however, we 

 heat the water containing a slice of beet up to a point which is sufficient to 

 kill the protoplasm, the red coloring matter in the cell-sap filters out through 

 the protoplasmic membrane and colors the water. If we heat a preparation 

 made for study under the microscope up to the thermal death point we can 

 see here that the red coloring matter escapes through the membrane into the 

 water outside. This teaches that certain substances cannot readily filter 

 through the living membrane of protoplasm, but that they can filter through 

 when the protoplasm is dead. A very important condition, then, for the suc- 

 cessful operation of some of the physical processes connected with absorption 

 in plants is that the protoplasm should be in a living condition. 



37. Osmose experiments with leaves. — We may next take the leaves of 

 certain plants like the geranium, coleus or iither plant, and place them in 

 shallow vessels containing water, salt, and sugar solutions respectively. The 

 leaves should be immersed, but the petioles should project out of the water or 

 solutions. Seedlings of corn or beans, especially the latter, may also be 

 placed in these solutions, so that the leafy ends are immersed. After one or 

 two hours an examination shows that the specimens in the water are still 

 turgid. But if we lift a leaf or a bean plant from the salt or sugar solution, 

 we find that it is flaccid and limp. The blade, or lamina, of the leal 

 droops as if wilted, though it is still wet. The bean seedling also is flaccid, 

 the succulent stem bending nearly double as the lower part of the stem is held 

 upright. This loss of tiurgidity is brought about by the loss of water from the 

 tissues, and judging from the experiments on spirogyra and the beet, we con- 

 clude that the loss of turgidity is caused by the withdrawal of some of the 

 water from the cell-sap by the strong salt solution. 



38. Now if we wash carefully these leaves and seedlings, which have been 

 in the salt and sugar solutions, with water, and then immerse them in fresh 

 water for a few hours, they will regain their turgidity. Here again we are led 

 to infer that the diffusion current is now inwartl through the protoi^lasmic 

 membranes of all the living cells of the leaf, and that the resulting turgidity 

 of the individual cells causes the turgidity of the leaf or stem. 



