58 &OTANV. ^ 



101. Stomata. If we examine carefully the row of epidermal 

 cells on the under surface of the leaf, we will find here and there 

 a peculiar arrangement of cells shown at figs. 47-49. This 

 opening through the epidermal layer is a stoma. The cells which 

 immediately surround the openings are the guard cells. The 

 form of the guard cells can be better seen if we tear a leaf in 

 such a way as to strip off a short piece of the lower epidermis, 

 and mount this in water. The guard cells are nearly crescent 

 shaped, and the stoma is elliptical in outline. The epidermal 

 cells are very irregular in outline in this view. We should also 

 note that while the epidermal cells contain no chlorophyll, the 

 guard cells do. 



102. The living protoplasm retards the evaporation of 

 water from the leaf. If we now take into consideration a few 

 facts which we have learned in a previous chapter, with refer- 

 ence to the physical properties of the living cell, we will be able 

 to give a partial explanation of the comparative slowness with 

 which the water escapes from the leaves. The inner surfaces of 

 the cell walls are lined with the membrane of protoplasm, and 

 within this is the cell-sap. These cells have become turgid by 

 the absorption of the water which has passed up to them from 

 the roots. While the protoplasmic membrane of the cells does 

 not readily permit the water to filter through, yet it is saturated 

 with water, and the elastic cell wall with which it is in contact 

 is also saturated. From the cell wall the water evaporates into 

 the intercellular spaces. But the water is given up slowly 

 through the protoplasmic membrane so that the water vapor 

 cannot be given off as rapidly from the cell -walls as it could if 

 the protoplasm were dead. The living protoplasmic membrane 

 then, which is only slow r ly permeable to the water of the cell- 

 sap, is here a very important factor in checking the too rapid 

 loss of water from the leaves. 



103. Communication through intercellular spaces. By an 

 examination of our leaf section we see that the intercellular 



