144 



Physics of the Soil. 



the plant. The chlorophyll grains can be effective in 

 breaking down the carbon dioxide only in comparatively 



txjj 



B 



Fia. 43. Diagram showing the mechanical action of guard cells In open 

 ing and closing breathing pores. The square shows the area of 

 under side of leaf containing an average of 209,000 breathing pores 

 or stomata. (From Irrigation and Drainage.) 



bright light and so, during cloudy days and at night, the 

 guard cells automatically change their form and close the 

 doors, reducing evaporation. Indeed they remain open 

 only when there is light enough to utilize it in decomposing 

 the carbon dioxide. 



175. Action of the Guard Cells. The opening and closing 

 of the guard cells is brought about by their peculiar shape 

 and changes in the amount of material they contain. 



Unlike the other cells in the epidermis of the leaf these 

 contain chlorophyll grains and are thus able to carry on the 

 process of developing plant food. The advantage of hav- 

 ing this work done here is to increase the osmotic pressure 

 through the rendering of the sap denser when the sun is 

 shining, thus distending the cells and changing their shape 

 so as to open the doors widest when the sun shines brightest, 

 as represented at A, Fig. 43. When night comes or it is 

 cloudy then the osmotic pressure forces the assimilated ma- 

 terial out of the guard cells faster than it is produced and 

 the walls collapse, taking the attitude represented at C and 

 in cross section at D, closing the opening. B and D are 

 cross sections of a pair of guard cells along the lines 1-2 

 and B shows how a full cell must pull the edges apart while 



