15 



more rows of large, tliiu-walled cells ^vliicli at the lower surface of the 

 leaf expand into a width of three or four cells and at the upper surface 

 terminate in a group of bulliforiu cells. 



The quantitj' and nature of h3'podernial fiber varies much in different 

 leaves. It always occurs, however, below all bundles in one or more 

 rows of cells and in several rows at the margin of the leaf and, except 

 below the miduerve, is separated from the bundles by chlorophyll- 

 bearing parenchyma. Hyi^odermal tiber above the bundles is repre- 

 sented b}^ cells which are small in diameter and have relatively large 

 cavities, but whose walls are firm (fig. 1, h, PI. VIII). The cells next to 

 the epidermis are usually more open, while the interior ones more 

 nearly approach oidinary fiber in character. The diameter of these 

 cells and the thickness of their walls vary considerably in different 

 leaves, and sometimes the tissue approaches colorless parenchyma 

 rather than fiber in appearance. Hypodermal fiber below the bundles 

 and at the margins usually shows an inner cellulose layer that almost 

 fills the cells, so that the cavity is sometimes barely distinguishable. 

 In less rigid leaves fiber below the bundles as well as that above is of 

 large lumeued cells, and variation in this respect is great. This is also 

 true of the leaves of the staminate i^lant. 



A median section (fig. 2, PI. YIII) has the V-shape usual to condupli- 

 cate leaves (compare fig. 3, PI. VIII). The leaf is thicker here, and the 

 ridges are consequently higher than in the basal section. The superior 

 epidermis and the arrangement and character of the chlorophyll tissue 

 is the same as iu the lower part of the leaf. The midrib is narrower and 

 very acute, and the lateral ridges also assume a triangular shape. The 

 bulliform cells are usually larger, and the bundles are all oval. In the 

 primary bundles all the cells of the sheath, including those above the 

 bundle, have very thick walls. Less fully developed bundles have two 

 oval vessels, and the phloem is separated from the chlorophyll tissue by 

 a row of sheath cells similar to those in the largest bundles. The large- 

 lumened, thin-walled cells are inclosed by the ring of ]ntted chlorophyll- 

 bearing cells. Their number and the tliiekness of their walls vary with 

 the thickness and rigidity of the leaf. The bands of colorless paren- 

 chyma between the ridges are not continuous from one face of the leaf 

 to the other, but are interrupted by a strand of fiber (fig, 4, h, PI, A'lII) 

 that is next the epidermis of the lower face and occupies the space 

 that, in the basal section, is filled by very thin-walled, large lumened, 

 colorless, parenchyma cells. The thickness of this fibrous strand varies 

 nmch in different leaves. It is sometimes represented by two or three 

 small cells with slightly thickened walls, and sometimes it forms a wide 

 band of very strong fibrous cells <)ccu]>ying a third of the thickness of 

 the leaf. The character of the fibrous cells is usually more uniform in a 

 median than in a basal section, although the cells on the upper surface 

 still show larger cavities than those below. From the middle to the apex 

 of the leaf the sections show^ successively fewer and narrower ridges, 



