ANATOMY OF YUCCA. 



293 



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

 inside the layer of j)alisade, and parallel to it, is a single layer of large, 

 thin-walled parenchyma cells containing chloroph}'!!, which represents 

 an inii^erfect i^arenchyma sheath to the mestonie ])undles. 



Colorless parenchyma in several layers which lie below the xmii-al 

 furrows and separate each two neighboring layers of palisado. 



Mestome bundles without a true niestome sheath, but willi Urn lep- 

 tome sui-rounded 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) ceils containing chlo- 

 rophyll, mostly somewhat elongated parallel to 

 tlie leaf axis, their walls, especially the arched 

 outer ones, greatly thickened and, together with 

 the miassive cuticle, exceeding the celllumen, the 

 radial walls not undulate; cuticle sharpl}^ differ- 

 entiated, beautifully stratified, divided b}^ per- 

 pendicular lamellsB 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 mesophjdl, each sur- 

 rounded by a parenchyma sheath. 



Stereome in massive groups on both tlie 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. 



Fig. U.— Yucca aloifolia — 

 leaf surface. Epidermis, 

 showing openings lead- 

 ing to the stomata. Scale 

 320. 



Fig. i2.— Yucca aloifolia- 

 a stoma. Cross- section 

 Scale 320. 



Leaf much like the preceding. 



Epidermis with larger (higher) cells; dorsal 

 surface bearing thick, rounded, 1-celled papilhe. 



Apical spine with exceedingly thick outer epidermis walls and 

 cuticle, these much higher than the celllumen, radial and inner walls 

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

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

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

 mestome bundle. 



