38 



PHYSIOLOGY. 



cellular spaces are not filled with water or cell-sap, but are filled 

 with air or some gas. Within the cells, on the other hand, we 

 find the cell-sap and the protoplasm. 



82. 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. 33-35- Tnis 



opening 

 through the 

 e p i de r m al 

 layer is a 

 stoma. The 

 cells which 

 i m mediately 

 surround the 



Fig. 36. 



openings are 

 the guard 



" '*' ""' rpJh The 



Portion of epidermis of ivy, showing irregular epidermal cells, stoma 0C " J * ilLC 



and guard cells. 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. 



83. 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 reference 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 mem- 

 brane 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 



