172 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 meso- 

 phyll chiefly in their spongy parenchyme cells and store consider- 

 able 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 

 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 

 Crustacese. 



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. 



Xerophytic. Centric laminae are produced gradually by an en- 

 croachment of the under on the upper surface, and the swelling of the 

 whole. In a completely centric leaf of the succulent kind, like that 

 of Sedum, the difference between the upper and lower surface is lost. 

 Stomata are found scattered over the entire epidermis. The bundles 

 are arranged in a circle, the mid-rib being in the center. A great 

 deal of mucilage is found stored in the central cells. In a typical 

 Xerophytic Centric leaf, like that of the Pine or Sansemera cylin- . 

 drica, the epidermis shows a tnick cuticle; the stomata are sunken in 

 cavities of the epidermis; the epidermis and leaf tissue are strength- 

 ened by scleroid bands in the centric mesophyll. 



D. Bifacial. Leaves with laminae which stand edge on in relation 

 to the sun's rays. The best illustrations are seen among dicotyle- 

 dons, such as Eucalyptus, Callistemnon, and other genera of Myr- 

 tacece. Both surfaces are similar, having stomata about equal in 



