32 FUNCTIONS OF EPIDERMIS. 



tinus, and Clerodendron fragrans. The cavities communicate with the 

 surface of the leaves by means of canals. 



Glands are occasionally sunk in the epidermis, so as merely to have 

 the apex projecting; at other times they lie below the epidermal cells, 

 as in the Myrtle, Orange, St. John's-wort, and Rue. 

 In the latter case they are sometimes called vesicular, 

 and are formed by cells surrounding cavities contain- 

 ing oil (fig. 89). When they occur in the leaves, 

 they give rise, when viewed by transmitted light, to 

 the appearance of transparent points or dots. Ver- 

 rucce, or warts, are collections of thickened cells on 

 the surface of plants, assuming a rounded form, and 

 containing starch or other matters. Lenticels, or Lenticular glands, are 

 cellular projections on the surface of the bark, arising from its inner 

 part. 



64. The Special Function* of the Epidermis and its appen- 

 dages, are to protect the parts beneath from various atmospheric 

 and meteorological influences. In plants growing in dry climates, 

 the epidermis is often very thick, and coated with a waxy secre- 

 tion, to prevent too great transpiration or exudation of fluids. In 

 those which inhabit humid places, the epidermis is thin and absorbent; 

 while in submerged aquatics, there is no proper epidermal cover- 

 ing. The stomata regulate the transpiration, opening and closing 

 according to the state of humidity and dryness of the atmosphere, 

 surrounding them. When a plant is growing vigorously, the constant 

 passage of fluids keeps the regulating cells around the stomata in a 

 distended state, and thus opens the orifice; whereas, when the circulation 

 is languid and the fluids are exhausted, the cells collapse and close 

 the opening. The opinion that the succulency of plants is a sort of 

 dropsical condition, caused by the absence of stomata to carry off the 

 fluids, has not been confirmed by observation. Hairs, according to 

 their structure, serve various purposes. Lymphatic hairs protect the 

 surface, and regulate evaporation. Plants thickly covered with 

 hairs, as Verbascum thapsus, have been knoAvn to resist well an 

 extended period of drought. Glandular hairs, and glands in general, 

 form secretions which are employed in the economy of vegetation, or 

 are thrown off" as excretions no longer fitted for the use of the plant 

 itself. Many of these secretions constitute important articles of materia 

 medica. Lenticels keep up a connection between the air and the inner 

 bark, and probably perform the function of stomata in the advanced 

 period of the growth of the plant. They are considered, by Decan- 

 dolle and others, as being the points where young roots are produced 



Fig. 89. Vesicular gland from Ruta graveolens, or Common Rue. gr, Gland formed by large 

 transparent cells, surrounding a central lacuna, I. e, Epidermis from upper surface of the taf 

 c, u c, Cells filled with Chlorophylle. 



