76 ADA HAYDEN 
Apocynaceae 
Apocynum cannabinum L. 
Habitat: Alluvial basin; wet soil. 
Orientation and arrangement: Opposite, short-stemmed; horizontal to ascending. 
Gross structure: Ovate to oblanceolate, glabrous or slightly pubescent. 
Histology: Bifacial (fig. lo, plate X). 
Upper and lower epidermis having thicker outer than inner walls. Lower epider- 
mis scalloped, the walls in the middle of the scallops being 3 times as thick as 
the inner walls; outer walls curved. 
Stomata small, slightly depressed; lower surface. 
Palisade parenchyma 3 layers; slender; space occupied equal to that of spongy 
parenchyma. 
Spongy parenchyma with moderate air space. 
Summary: The thickenings of scallops seem practically equivalent to a uniformly 
thickened cuticle, for the thin places are opposite walls. This leaf shows indications 
of abundant water with adequate conservation facilities and a tendency to endure 
drought. 
Asclepiadaceae 
Asclepias verticillata L. 
Habitat: Alluvial basin. Basal slopes or low, level, moist areas. 
Orientation and arrangement: Whorled, somewhat appressed toward the stem in an 
upward direction. 
Gross structure: Linear with revolute margins, glabrous. 
Histology: Bifacial (fig. 11, plate X). 
Upper epidermis twice as thick as lower, outer wall thicker than inner in both 
cases; walls curved. 
Stomata on under surface, with thick lower walls; level with epidermis. 
Palisade parenchyma two layers; broad, large, wedge-shaped cells; occupies 3/8 
parenchyma space. 
Spongy parenchyma twice as much as palisade, prominent air space; occupies 5/8 
parenchyma space. 
Summary: The slender leaves, the reflexed edges, and the thick cuticular wall indicate 
protective, water-retentive characters, but the abundant photosynthetic tissue with 
much air space would indicate sufficient available water. 
Labiatae 
Physostegia virginiana (L.) Benth. 
Habitat: Alluvial basin; wet soil. 
Orientation and arrangement: Horizontal, opposite; sessile. 
Gross structure: Lanceolate to oblong; glabrous. 
Histology: Bifacial (fig. 13, plate XI). 
Epidermal cells moderate-sized; outer walls both about two times as thick as 
inner walls. 
Stomata small; on the level with lower surface of epidermis. 
Palisade parenchyma, cells broad; two layers, occupying about 1/2 parenchyma 
space. 
Spongy parenchyma with large cells; much air space. 
Summary: The surface stomata, the relatively thin-walled epidermis, the large-celled 
loose structure indicate photosynthetic activity and abundance of water without 
much tendency toward its conservation. 
Mentha arvensis var. canadensis (L.) Briquet. 
Habitat: Alluvial basin; damp soil. 
Orientation and arrangement: Horizontal; opposite, lower leaves petioled. 
