13 



the differences iu structure that accompany the outward changes of 

 form and texture from base to apex. Tlie lower part of tlie l)lade is 

 inclined to be invohite rather than conduplicate, but the central por- 

 tion is closely conduplicate when dry (probably always so) and the 

 leaf thence tapers to a sharp, rigid, thorn-like apex. 



A basal transverse section of a leaf of the ? plant otJ.inlosa (tig. 1, 

 PI, YIII) has a slightly curved form iu accordance with the involute 

 character of the leaf at this point. The midrib differs from the other 

 large veins only iu the greater quantity of hypodermal fiber that 

 accompanies it on the lower face. The upper face of the leaf has deep 

 narrow furrows with rounded or nearly square ridges intervening. 

 The lower face has no furrows but is sometimes slightly undulate. 



On the upper leaf surf aee the band of epidermis covering the fibrous 

 tissue at the summit of each ridge is composed of several rows of alter- 

 nately long and short cells interrupted by numerous, thick-walled uni- 

 cellular prickles. At each side of this band are several rows of cells 

 (figs. 1 and 2, «, PI. VIII) that cover the sides of the ridges and border 

 immediately on chlorophyll-bearing parenchyma. They have numer- 

 ous exodermal expansions that take the form of bluntly conical or 

 somewhat capitate projections of some portion of the outer walls of 

 the rectangular cells. These protuberances are often irregularly 

 grouped around the stomata (fig. 1, .v/., PI. VIII) which occur in one or 

 more longitudinal rows on each side of the ridges. Short, nearly 

 spherical, two-celled hairs occur in the rows of cells marking the border 

 between the first and second named bands of the epidermis. The long 

 unicellular hairs that cover the inner surface of the leaf at its base 

 spring from circular mound-like groups of deep and narrow colorless 

 cells. These occur among the bulliform cells and at the margins of 

 the leaf. Owing to the depth of the furrows, a surface view of the 

 bulliform cells is difficult to obtain, but a transverse section (fig. 1, B, 

 PL VIII) shows them to be in from three to five rows at the base of each 

 furrow. The lateral ones are not distinctly different from other epi- 

 dermal cells. 



On the lower surface th^i epidermal cells (fig. 1, Z>, PI. VIII) covering the 

 bands of hypodermal fiber are narrower than those over the other 

 tissues. They are bordered on each side by one row of cells interrupted 

 by stomata. The epidermal cells covering the colorless cells between 

 the veins are in several rows and are wider than those over the fiber 

 bands. Their walls are undulate, and long and short cells alternate. 

 Iu transverse section the epidermal cells of the lower face are remark- 

 able for their large cavities. Although those covering the fiber bands 

 are smaller than the others, yet throughout the leaf and even at the 

 tip which consists wholly of sclerenchymatous fiber (fig. 5, PI. VIII), the 

 large lumina of the epidermal cells are conspicuous. Their outer Avails 

 are thickened and the surface of the leaf is very smooth and even. In 

 the median portion of the leaf there are no exodermal expansions, but 



