ANATOMY OP YUCCA. 



293 



the leaf surface and extend nearly to the summit of the ventral ridges; 

 inside the layer of palisade, and parallel to it, is a single layer of large, 

 thin-walled parenchyma cells containing chlorophyll, which represents 

 an imperfect parenchyma sheath to the mestome bundles. 



Colorless parenchyma in several layers which lie below the ventral 

 furrows and separate each two neighboring layers of palisade. 



Mestome bundles without a true mestome sheath, but with the lep- 

 tome surrounded by an unbroken ring of mes- 

 tome parenchyma having small cells with thick 

 porous walls. 



Yucca aloifolia L. 



Leaf isolateral, thick, especially toward the 

 base, ending in a rigid apical spine. 



Epidermis (figs. 41, 42) cells containing chlo- 

 rophyll, mostly somewhat elongated parallel to 

 the leaf axis, their walls, especially the arched 

 outer ones, greatly thickened and, together with 

 the massive cuticle, exceeding the cell lumen, the 

 radial walls not undulate; cuticle sharply differ- 

 entiated, beautifully stratified, divided by per- 

 pendicular lamellae corresponding to the radial cell walls; stomata 

 deeply sunken, lying beneath the cuticle at the bottom of urn-shaped 

 passages whose outer orifice is quadrangular with raised, cushion-like 

 borders, the ridges of exit at bottom of the pore very acute ; papillae 

 none (perhaps present in younger leaves). 



Interior of the leaf occupied by homogeneous, thin-walled paren- 

 chyma, which, toward the apex of the leaf, contains chlorophyll in 

 its entire thickness. 



Mestome bundles lying in several rows in the mesophyll, each sur- 

 rounded by a parenchyma sheath. 



Stereome in massive groups on both the lep- 

 tome and the hadrome side of the bundles, espe- 

 cially strong on the hadrome side. Small bun- 

 dles of stereome, each with a parenchyma sheath, 

 are scattered among the mestome bundles. 



Yucca gloriosa L. 



PlO. 41. — Yucca aloifolia — 

 leaf surface. Epidermis, 

 showing openings lead- 

 ing to the atomata. Scale 

 330. 



FIG. 42, — Yucca aloifolia- 

 a stoma. Cross section 

 Scale 330. 



Leaf much like the preceding. 



Epidermis with larger (higher) cells; dorsal 

 surface bearing thick, rounded, 1-celled papillae. 



Apical spine with exceedingly thick outer epidermis walls and 

 cuticle, these much higher than the cell lumen, radial and inner walls 

 also much thickened; next a hypodermal layer of thick-walled col- 

 lenchyma; then 1 or 2 layers of thick- walled collenchymatic tissue; 

 and, finally, a dense mass of stereome, inclosing a small central 

 mestome bundle. 



