NATURE OF PLANTS 



35 



plants, but in leaves exposed to intense sunlight and a hot and 

 dry air, the epidermal cells become greatly thickened and often 

 of two or more rows, while the cuticle may often form the larger 

 part of the outer cell wall or even extend in between the cells 

 (Fig. 23, c). Cell walls that are filled with cutin in this manner 

 are said to be cutinized. Such features are particularly notice- 

 able in desert plants, in long-lived leaves of many 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, stereome (Fig. 23, st). Coatings of wax, mucilage and 



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. 



lime are also frequently developed upon the cuticle to further 

 re-inforce 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 

 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 



