36 FUNCTIONS OF EPIDERMIS. 



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

 as in the Myrtle, Orange, St. John's-wort, and Eue. In the latter 

 case they are sometimes called vesicular, and are formed by cells sur- 

 rounding cavities containing oil (fig. 95). When 

 they occur in the leaves, they give rise, when 

 viewed by transmitted light, to the appearance 

 of transparent points or dots. Verrucce, 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 

 Fig.'gs. glands, are cellular projections on the surface of 



the bark, arising from its inner part. Trecul says 

 that lenticels result from the formation of corky matter under decayed 

 or decaying tissues, the corky particles surrounding sub-stomatic cavi- 

 ties. The corky matter protects the internal tissue from injurious 

 atmospheric influence. Other lenticels are simply cracks of the epi- 

 dermis before the production of cork or periderm, while a third set 

 are produced on the surface of a peridermic layer. 



The Special Functions of the epidermis and its appendages 

 are to protect the parts beneath from various atmospheric and meteoro- 

 logical influences. In plants growing in dry climates, the epidermis 

 is often very thick, and coated with a waxy secretion, 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 covering. The stomata regulate 

 the transpiration ; opening and closing, according to the state of humid- 

 ity and dryness of the atmosphere surrounding them. When a plant 

 is growing vigorously, the constant passage of fluids keeps the regu- 

 lating 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 pur- 

 poses. Lymphatic hairs protect the surface, and regulate evaporation. 

 Plants thickly covered with hairs, as Verbascum Thapsus (Great 

 Mullein), have been known to resist an extended period of drought. 

 When organs become abortive they sometimes assume the form of hairs. 

 Glandular hairs, and glands in general, form secretions which are em- 

 ployed in the economy of vegetation, or are thrown off as excretions 

 no longer fitted for the use of the plant itself Many of these secre- 

 tions constitute important articles of materia medica. Lenticels keep 



Kg, 95. Vesicular gland from Euta graveolens, or Common Eue. g. Gland formed by 

 large transparent cells, surrounding a central lacunaj I. e. Epidermis from upper surface 

 of tlie leaf. uc,uc, Cells filled with Chlorophyll. 



