NATURE OF PLANTS 33 



The stomata are also developed as to meet the conditions under 

 which the plant grows. In dorsiventral leaves they are more 

 numerous on the under side of the leaves because they are less 

 liable to be filled with water by rains and with dust which would 

 prevent the interchange of gases. The plugging of the stomata 

 by dust is one of the causes of the sickliness of plants in homes. 

 The arrangement of the stomata on the under side of the leaf is 

 also of especial advantage because the direct light does not fall 

 upon them and cause an excessive loss of water. The stomata 

 of floating leaves, however, are upon the upper surface and their 

 stoppage with water is prevented by waxy coatings, as can be 

 easily demonstrated by dipping a leaf of a water lily or spatter 

 dock in the water. When the leaf is removed the water runs 

 off of the waxed surface without wetting it. Some leaves are 

 more or less erect, as the cattails, rushes and grasses, and these 

 have the stomata developed more or less evenly on both surfaces. 

 Doubtless the intense light of midday is not beneficial to these 

 leaves and the blade of the leaf is consequently placed parallel 

 with the sun's rays. This arrangement of the leaf permits a 

 direct illumination of the leaf only in the morning and afternoon 

 when the intensity of the light is feebler. Leaves develop, fewer 

 stomata in a dry atmosphere and intense light than in moist and 

 shady places. Light and moisture also influence the position of 

 the stomata in another interesting way. Plants living in the 



FIG. 24. Cross-section of a leaf of the inch plant, Tradescantia, show- 

 ing the delicate character of the cells and the raised stoma of this shade- 

 loving plant. 



