GOSSYPIUM 435 



lower epidermis, 175; hypocotyl and stem, io. The stoma is 

 surrounded by two small guard cells with conspicuously thickened 

 outer, inner, and radial walls. In the palisade region, the cells are 

 very much elongated so that this layer comprises approximately 

 one-third to one-half the thickness of the leaf. The underlying 

 spongy cells are more loosely arranged with numerous intercellular 

 spaces; and, adjacent to the enlarged midrib, lysigenous glands 

 occur similar to those in the cortex of the stem. In some instances, 

 these are very large and extend from the lower epidermis into the 

 palisade layer. The lower epidermis resembles the upper except 

 that the cells are more irregular. 



The main veins have rib-like projections on the adaxial surface 

 consisting of strands of thick-walled mechanical cells which are 

 separated from them by several layers of thin-walled parenchyma. 

 The vascular bundle is collateral, resembling that of the stem in the 

 seriate arrangement of its xylem cells, in the well-defined cambium, 

 and the rather regularly arranged groups of phloem elements. On 

 the abaxial surface, there is a zone of compact parenchyma forming 

 a prominent ridge beneath each of the larger veins, and this is 

 usually reinforced by two or three layers of thick-walled cells. 



Nectaries occur on the abaxial surface of the main ribs of the 

 leaf in all cultivated varieties grown in the United States with the 

 possible exception of Gossypium tomentosum Nutt. Tyler (35) 

 reports that the number varies from one to five with one to three 

 being most common. They appear as small, shallow, oval, pear- 

 shaped, or sagittate pits that may be naked, although, in many 

 varieties, there are stellate hairs protruding from the surrounding 

 surface. Club-shaped papillae arise from the floor of the pit, each 

 consisting of several glandular cells. (Fig. xi4. A, B.) Reed (16) 

 has described the development of the nectaries in Gossypium 

 hirsutum, and reports their occurrence on the cotyledons as well 

 as on the other foliage leaves. The papillae are epidermal in ori- 

 gin, and the first walls that come in are transverse so that short 

 papillae are formed consisting of two to four cells. This is fol- 

 lowed by a vertical division of the terminal cell, and each resulting 

 daughter cell again divides, the new wall lying at right angles to 

 the first cross wall. Periclinal divisions of these four terminal 

 cells result in the formation of four central and four peripheral cells. 

 Finally, each of the external cells divides so that a peripheral layer 

 of eight cells is formed. 



