88 General Botany 



cells of the plant are in contact either with water or with other 

 cells containing water. Cells may, therefore, take up water 

 either from adjacent cells or from their surroundings. 



The conditions for osmosis as it occurs in plants, then, include 

 a differentially permeable membrane between two masses of 

 water, one of which is capable of developing a higher osmotic 

 pressure than the other. The water passes from the mass of 

 water or adjoining cell in which it is most concentrated to the 

 cell in which dissolved substances are most concentrated and the 

 water is least concentrated. 



Turgor. When cells contain dissolved substances, or sub- 

 stances that swell greatly in water, osmosis and imbibition lead 

 to the taking up of water and the stretching of the cell walls. 

 A cell that is thus distended is said to be turgid. Cells with an 

 inadequate water supply may be only partly filled, and the cell 

 walls are consequently not stretched. Cells in this condition are 

 flaccid. A condition of turgor is present in actively growing 

 tissues. Cells are flaccid when a plant shows the familiar signs 

 of wilting. 



The movement of the guard cells of stomata (page 74) is 

 brought about by turgor pressure, being open when the pressure 

 is high and closed when the pressure is low. The changes in the 

 position of leaves possessing a pulvinus are also due to changes 

 in the turgidity of a part of the cells of the pulvinus. 



Materials move by various combinations of physical processes. 

 The physiological processes involved in the movement of materials 

 in plants are various combinations of the four physical processes 

 briefly outlined. Oxygen and carbon dioxide move into a cell 

 by solution and diffusion. They pass out of a cell by diffusion 

 when the cell contains a greater concentration than the surround- 

 ing air or the adjacent cells. Water passes from one cell to 

 another by diffusion, particularly the forms of diffusion called 

 imbibition and osmosis. As a result of imbibition and osmosis, 

 pressure develops in the cell, stretching the walls and resulting 



