HISTOLOGY OF PERSEA I'UBESCENS. 489 



Palisade in two layers, the cells rather low. Pneumatic tissue only 

 moderately open. Oil reservoirs numerous in the mesophyll. 



CoUenchymatic tissue strongly developed above the large veins, 

 especially the mid vein; less strongly developed beneath them. 



Mestome bundle of the large veins arranged in a more or less perfect 

 cylinder. 



Stereome completely surrounding the mid vein, especially strong 

 beneath it, where it is separated from the collenchymatic tissue by a 

 few layers of thin-walled colorless parenchyma. Narrow plates of 

 stereome support the smaller veins, extending through the entire 

 thickness of the leaf. 1 Stereome is also rather strongly developed in 

 the leaf margins. 



Persea pubescens (Pursh) Sargent. 



Mixed and Hygrophile Forest formations. 



Leaves thick, more or less persistent, bifacial, dark green and some- 

 what shining above, somewhat glaucous and, especially when young, 

 short-pubescent beneath. Structure in many respects similar to that 

 of Magnolia glauca. 



Epidermis: Ventral, cells tabular, the cuticle considerably thick- 

 ened, the radial walls undulate. Dorsal, cell walls thinner, cuticle 

 covered with a granular coaling of wax, radial walls not undulate. 

 Stomata confined to the dorsal surface, exceedingly numerous,- small, 

 lying in all directions, level with the surface, each bordered by usually 

 five unmodified epidermis cells. I lairs con fined to the dorsal surf ace in 

 older leaves, chiefly along the veins, long, sharp-pointed, with thick 

 cuticle, unicellular. 



Palisade in two layers, the outer more compact, interrupted over 

 the veins by stereome. Pneumatic tissue lacunous. 



Oil reservoirs in the chlorenchyma, large, spheroidal. 



Larger veins strengthened above and below the mestome by massive 

 groups of thick-walled stereome. Smaller veins supported by thin 

 plates of thin- walled stereome. 



Several layers of rather thick- walled colorless parenchyma separate 

 the stereome supporters beneath the veins from the epidermis. 



As would be expected, this species, which grows where air and soil contain 

 abundant moisture at almost all seasons, exhibits a much less pronounced xero- 

 phytic structure than is found in others of the Lauraceae whose leaves are longer- 

 lived and are adapted to a drier climate and soil. Lauras ndbUis, for example, 

 has an extremely compact palisade tissue of very long and narrow cells, and 

 its stomata are placed at the bottom of cavities of which the external orifice is 

 very small." 



1 1n M. tripetala (M. umbrella ) chlorenchyma occurs above and below the stereome 

 supporters of the small veins, although that species has a leaf considerably thin- 

 ner than that of M. Virginia iki. ( Vesirue, Nouv. Archiv. Mus.. st'-r. 2, vol. 4, p. 137.) 



'-'Lalanne (Feuilles persistantes, p. <S2) finds the presence of a very large num- 

 ber of stomata on the under surface to be characteristic of leaves which are cori- 

 aceous, even when not " evergreen." 



s Lalanne, op. cit., pp. GG to 63. 



