OF THE EASTERN UNITED STATES 383 



its terminal cell or one of its middle cells specialized as a gland and 

 containing oil. Isolated epidermal cells of the ordinary kind often 

 become enlarged and filled with oil and so also function as glands. 



Comptonia asplcnifolia, (L.) Alton 

 The upper epidermis of the leaf of this plant consists of a layer of 

 cells whose external walls are slightly arched outward and covered by a 

 granular cuticle. The vertical walls of these cells are curvilinear. Over 

 the veins the cells become rectilinear and elongated in the direction of 

 the vein bundles. The mesophyll shows dorso-ventral differentiation 

 into a broad palisade zone and a narrow spongy parenchyma region. 

 The palisade zone consists of usually two layers of somewhat loosely 

 arranged columnar shaped cells, the outer layer having its elements 

 much broader than the inner one. The spongy parenchyma region is 

 composed of irregular shaped loosely arranged cells surrounding inter- 

 cellular-air-spaces. Rosettes of calcium oxalate are common in many 

 cells of the mesophyll. The vascular systems of the mid-rib and smaller 

 veins run through this region. That of the mid-rib is supported above 

 and below by an arc of sclerenchyme. Those of the smaller veins are 

 connected with the upper and lower epidermis by means of elongated 

 hypodermal elements, some of which contain rosettes of calcium oxalate. 

 The lower epidermis consists of a layer of cells with a granular cuticle. 

 Their external walls are arched outward in a more prominent manner 

 than those of the upper epidermis. The cells of this epidermis conse- 

 quently are distinctly papillose in character. Their vertical walls are 

 somewhat more curvilinear than those of the upper epidermis. Numer- 

 ous stomata are scattered irregularly amongst the epidermal cells. These 

 are surrounded by neighboring cells. Both upper and lower epidermis 

 show four kinds of hairs. Of these, two are simple and two glandular. 

 One type of simple hairs and by far the more common is composed of 

 a single cell whose wall is sclerified throughout. At its base, as also 

 shown by Chevalier, (7, p. 118) sclerenchyme threads extend in between 

 surrounding cells in order to strengthen its insertion. The second type 

 of simple hair is furchate and really con'Sists of two hairs arising from 

 contiguous basal cells whose adjoining margins have become soldered 

 at the base. The first type of glandular hair is also seen in M. Gale 

 and consists of 3-4 superimposed cells whose terminal cell or one of its 

 median cells becomes enlarged and filled with oil. The second type 

 consists of a stalk of 3-4 cells bearing on its summit a several celled 

 spherical head which contains oil. 



