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PHARMACEUTICAL BOTANY 
the lower surface or present more or less equally on both surfaces, 
where they are sunken in depressions. They may be either firm, 
leathery, tough, fibrous, or may become swollen up in their mesophyll 
chiefly in their spongy parenchyme cells and store considerable 
mucilage. Examples: Yucca, Ficus, Aloe, Agave. Succulent forms 
like Aloe generally possess a thin but tenacious cuticle. 
4. Hydrophytic. — All gradations are seen. In pond plants, such 
as the Water Lily, the leaves have long split petioles which bring 
the blade up to the surface of the water. The stomata are entirely 
Fig. 60. — Photomicrograph of dorsoventral leaf*of Myrica showing upper epi- 
dermis (u.e.), palisade parenchyma (p.p.), vein (v), spongy parenchyma (s.p.), 
lower epidermis (l.e.), glandular-hairs (g) and stoma (5). (Highly magnified.) 
on the upper surface. In Ranunculus, the lower leaves are cut up 
into filiform segments. These are devoid of stomata. Their meso- 
phyll is soft, open, and spongy. The epidermis is quite thin. The 
upper leaves are floating, trilobed, and have stomata only on their 
upper surface. In Utricularia, some of the filiform submerged leaves 
are modified into bladders which trap insect larvae and smaller 
Crustaceae. 
B. Convergent. — In Phormium tenax, the base of the blade is 
sheathing, it then converges and opens out above. In the various 
species of Iris the petiole is sheathing, the upper part being fused 
(mostly seen in monocotyls). 
C. Centric. — Succulent. — Nearly always associated with Xero- 
phytes. 
