42 



PHYSIOLOG V. 



Fig. 53. 

 Portion of epidermis of ivy, showing in- 

 and guard cell 



:giilar epidermal cells, stoma ^ 



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 carefirlly the row of epidermal 

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

 a peculiar arrangement of cells shown at figs. 51, 52. This 



opening 

 through the 

 e p i de rinal 

 layer is a 

 stoma. The 

 cells which 

 i m mediately 

 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 tins 

 in water. The guard cells are nearly crescent shaped, and the 

 stoma is elliptical in outline. The epidermal (,:ells are very 

 irregular in outline in this view. We should also note that while 

 the epidermal cells contain no chlorophyll, the guard cells do. 



82a. In the ivy leaf the guard cells are quite plain, but in most 

 plants the form as seen in cross-section is irregular in outline, as 

 shown in fig. 530, which is from a section of a wintergrcen leaf. 

 This leaf is interesting because it shows the characteristic struc- 

 ture of leaves of many plants growing in .soil where absorption of 

 water by the roots is difficult owing to the cold water, acids, or 

 salts in the water or soil, or in dry soil (see Chapters 47, 54, 55). 

 The cuticle over the upper epidermis is quite thick. This 

 lessens the loss of water by the leaf. The c<mipact palisades of 

 cells arc in two to three cell layers, also rc( hieing the loss of water. 



83. The living protoplasm retards the evaporation of water from the 

 leaf. — If we now take into tonsideration a few faets wltich we have learned 



