NATURE OF PLANTS 



37 



evergreens, as laurels and rhododendrons, in the needles of cone- 

 bearing trees, and in all plants that are exposed to the hot, dry air 

 of summer or the drying winds of winter. Such leaves may be 

 further protected and strengthened by thick-walled elongated 

 cells, sterome (Fig. 23, st). Coatings of wax, mucilage and 

 lime are also frequently developed upon the cuticle to further 

 reinforce the impermeability of the epidermis. 



The stomata are also so 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 



Fig. 23. Cross-section of the outer cells of a leaf of pine, showing the firm 

 character of the outer cells of the tough leaf: s, stoma; e, epidermis; c, cuticle; 

 st, stereome; m, mesophyll cells. 



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 



