246 PHARMACEUTICAL BOTANY 
stipules unite at their margins, grow up and sheathe the stem, 
forming a legging-like sheath, as in many species of the Poly- 
gonacee such as Buckwheat, Rhubarb, Yellow Dock, Knot Weeds, 
etc. The sheath formed is called an OcHREA. 
Mobptrriep StreuLEs.—In some plants such as the Locust and 
several other trees and shrubs of the Legume family, the stipules 
become modified for defensive purposes as spines or prickles. 
In the Sarsaparilla-yielding plants and other species of the genus 
Smilax they undergo modification into tendrils which are e useful i in 
climbing. (See Fig. 366.) 
THe LAMINA 
This, as was previously indicated, represents an expansion of 
the tissues of the petiole, but in sessile leaves is directly attached ~ 
to the stem and so is a direct stem outgrowth. 
Mope or DEVELOPMENT OF THE LAMINA OF LEAvEs.—The 
lamina of leaves develops in one of six ways. 
1. Normal or Dorsiventral. 
Convergent. 
Centric. 
Bifacial. 
Reversed. 
. Ob-dorsi-ventral. 
The first four will only be considered. 
A. DorsIvENTRAL (the commonest). 
(a) DorstvENTRAL UmsBropuytic.—Flattened from above 
downward. Plants with such leaf blades tend to grow in the 
shade. 
(6) DorstvENTRAL Mesopuytic.—Similar to the former, but 
plants usually grow directly in the pporhe and exposed to sunlight 
and winds. 
(c) DorstvenTRAL XeEROpHyTIC.—Similar to former, but — 
plants not only grow exposed, but exposed to hot desert conditions 
or to cold vigorous conditions. 
(7) DorstvENTRAL Hypropuytic.—All transitions occur 
between typical mesophytic forms to those of marshy places, to 
swamps and borders of streams and finally with leaves wholly 
emersed, the last a completely hydrophytic type. 
Auk WN 
