ELEMENTARY STRUCTURE. 45 



on the upper surface. They occur also on the young green 

 shoots of plants and on the parts of the flower, and in the 

 interior of the fruit of the Wall-flower, and on the seed of 

 the walnut. In such plants as have no true leaves, as the Cac- 

 taceae, they abound upon the green succulent stems. They 

 are commonly only found on those parts which are furnished 

 with a true epidermis, and are accordingly absent in roots and 

 all submersed parts of plants. They are also absent in pale 

 parasitical plants, from the epidermis of plants growing in dark- 

 ness so as to be blanched, and from the ribs of leaves. 



The exact origin and mode of development of stomata is not 

 clearly ascertained. By Mohl, and other authors, the stomatal 

 cells are described as originating from one of the cells situated 

 below the epidermis, rising into a space formed by the separation 

 of the epidermal cells at the points where stomata occur. 

 Nageli, and others again, describe the stomatal cells as being 

 formed originally out of true epidermal cells, which are subse- 

 quently placed on a level with these, or become pushed down- 

 wards or upwards, according to the ultimate position of the 

 stomata We hold the opinion taken by Mohl as to their origin, 

 which we think is proved by the frequent occurrence of chloro- 

 phyll in the stomatal cells, such substance not being found in 

 the epidermal cells. While observers differ as to the origin of 

 the stomatal cells, they are all agreed as to their general mode 

 of formation from the particular cells, each constituent cell 

 which forms the stoma becoming divided into two or four sto- 

 matal cells, according to the usual mode of cell-division, to be 

 afterwards described (see p. 58): these ultimately separating 

 from each other in the line of partition so as to leave an orifice 

 or interspace between, and thus, accordingly as we have the 

 division of the cells which form the stoma into two or four 

 parts, so we have the orifice bordered by two or four stomatal 

 cells respectively. 



5. Appendages of the Epidermis.— Upon the surface of the 

 epidermis, or in the sub- epidermal tissue, there are frequently 

 to be found certain structures consisting of cells variously com- 

 bined, and containing various substances, which are termed 

 collectively Appendages of the Epidermis. We shall treat of 

 these under the two heads of Hairs and Glands. 



1. Hairs. — These are thread-like prolongations externally of 

 the epidermal cells and covered by cuticle (figs. 79, g, and 84). 

 They may either consist of a single cell, when they are called simple 

 (figs. 96, 97, and 98) , or of several cells, when they are called com- 

 pound (figs. 103 and 104). Simple hairs may be undivided (fig. 

 96), or forked (fig. 97), or branched (fig. 98). A very beautiful 

 form of a simple hair is that called Stellate, as seen in Deutzia 

 scabra, Alyssum, &c. (fig. 99); this is formed by a cell dividing 

 horizontally into a number of parts which are arranged in a star- 



