RELATION OF STOMATA TO ENVIRONMENT 



129 



stomata are all on the lower side, and still fewer instances, as 

 in the water-lily, where the stomata are practically all on the 

 upper side. Where the leaves hang downward, like those of 

 some poplars, or stand upright, as in the grasses, the number 

 of stomata is about equal on both 

 sides. 



Often when the stomata occur on 

 one side only they make up in fre- 

 quency there for the lack on the 

 other side. Thus, per square 

 millimeter of surface, Nymphsea 

 alba has 460 stomata on the upper 

 side and none on the under; Pirus 

 malus has none on the upper side 

 and 246 on the lower; while 

 Triticum sativum has 47 on the 

 upper and 32 on the lower side. 



The Relation of Stomata to 

 the Environment. — In desert re- 

 gions and in places where plants 

 at certain seasons are in danger of 

 suffering from scarcity of water it 

 has been found advantageous to 

 plants to equip the stomatal ap- 

 paratus with devices that retard 

 evaporation while allowing the 

 diffusion of gases into the leaf. 

 A common plan is to sink the stomata below the level of the 

 general epidermis, so that each is at the bottom of a pit where 

 the wind scannot sweep away at once the water vapor as 

 fast as it is transpired. Sometimes the outer wall of the 

 guard cells is elevated in the form of a crater, or an elevated 

 border of wax may serve to maintain a quiet atmosphere just 

 above the stoma. However the pit or crater is produced it is 

 sometimes made more effective by outgrowths that partially roof 

 it over, as in Fig. 66. 

 9 



Fig. 66. — A, depressed stoma of 

 the under side of a leaf oi Amherstia 

 nobilis. B, depressed stoma of 

 Hakea suaveolens. g, strands be- 

 neath the guard cells; d, outer, and e 

 inner, cavities. (After Haberlandt.) 



