PHYSIOLOGY 



463 



surface of equal area. In this way it is possible to explain the 

 power of leaves to absorb so large a quantity of C0 2 from the atmo- 

 sphere, where it is present in such small proportion. 



Imbibition 



The imbibition of water is one of the most marked characteristics 

 of organized substances, and plays a most important part in the 

 nutrition of protoplasm. In their normal condition, both cell-wall 

 and protoplasm contain very large amounts of imbibed water, the 

 amount in living protoplasm being so great as to render the proto- 

 plasm of semifluid consistency. The mechanism of transport for 

 the food substances from cell to cell within the plant is directly 

 dependent upon this property of imbibition, though other factors are 

 connected with the transfer of watery solutions through the special 

 conducting tissues. 



A 



FIG. 450. A, diagram to show the mechanism of the opening of a stoma in Helle- 

 borus sp., seen in transverse section. The heavy lines indicate the position of the 

 guard-cells when open, the dotted lines the position when closed. (After SCHWEN- 

 DENER.) B, Galtonia candicans, surface. view of a stoma, showing a turgid 

 guard-cell, G 1 , and a guard-cell contracted by plasmolysis, G. (After LEITGEB.) 



The Mechanics of Absorption 



The cellulose cell-wall, when saturated with water, is more perme- 

 able than the plasma-membranes lying within it. In the typical 

 cell there are two of the latter, the ectoplasm, or bounding layer 

 immediately within the cell-wall, and the endoplasm, which bounds 

 the central vacuole. That these plasma-membranes, in the living 

 cell, are less permeable than the cell-wall, is shown in cells with col- 

 ored cell-sap, like those in the Beet root, or in many red leaves. 

 The pigment is dissolved in the cell-sap, and does not pass through 

 the bounding membranes so long as the cell is alive. If the proto- 

 plast is killed, however, the colored cell-sap diffuses through the 

 dead plasma-membranes, and then readily passes out of the cell 

 through the cell-wall. So, also, if the cell is plasmolyzed with a 



