TUB, EPIDERMAL SYSTEM. 93 



123. The change in the form of the epidermal cells is 

 due to the mode of growth of the organ of which they form 

 a part; the lateral and longitudinal growth of an organ 

 causes a corresponding extension and consequent flattening 

 of the cells ; if the growth has been mainly in one direction, 

 as in the leaves of many Monocotyledons, and the young 

 shoots of many Dicotyledons, or if the growth in two direc- 

 tions has been regular and uniform, as in the leaves of some 

 Dicotyledons, the cells are quite regular in outline ; where, 

 however, the growth is not uniform the cells become irregu- 

 lar, often extremely so (Fig. 89, page 100). 



124. The thickening of the walls is greatest in those 

 plants and parts of plants which are most exposed to the dry- 

 ing effects of the atmospheie. It consists of a thickening of 

 the outer walls, and frequently of the lateral ones also. The 

 outer portion of the thickened walls is cuticularized, and 

 this, by a subsequent stratification and lamellation, is separ- 

 ated as a continuous pellicle, the so-called cuticle. 



125. The cuticle extends uninterruptedly over the cells, 

 and maybe readily distinguished from the other portions 

 of the outer epidermal walls. It is insoluble in concen- 

 trated sulphuric acid, but may be dissolved in boiling 

 caustic potash. Treated with iodine it turns a yellow or 

 yellowish brown color. A waxy or resinous matter is fre- 

 quently developed upon the surface of the cuticle, constitut- 

 ing what is called the Uoom of some leaves and fruits. De 

 Bary* distinguishes four kinds of waxy coating, as follows : 

 (1) continuous layers or incrustations of wax e.g., on the 

 leaves and stems of purslane, the leaves of Fuchsia, yew, the 

 stems of the wax palms (Ceroxylon], etc. ; (2) coatings com- 

 posed of multitudes of minute rods placed vertically side by 

 side upon the cuticle e.g., on the stems of sugar cane, 

 Coix lachryma, and some other grasses ; (3) coatings made 

 up of minute rounded grains in a single layer e.g., on the 

 leaves of the cabbage, onion, tulip, clove-pink (Dianthus 



* " Vergleichende Anatomic der Vegetationsorgane der Phaneroga- 

 men und Fame," 1877, p. 87, where figures of several of these kinds 

 are given. 



