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1 68 PHARMACEUTICAL BOTANY
This is most commonly the rule, but when the plant is exposed to
xerophytic conditions, as the Acacias of Australia, the stalk, instead
of being cylindric or sub-cylindric, becomes flattened from side to
side, until there is produced a bifacial vertically placed petiole, with
a large green surface that wholly takes the place of the lamina.
The petiolar structure in primitive types of Dicotyls resembles
that^seen in Monocotyls except that the bundles are Cardizem La more con-
densed side by side. In these the petiole is somewhat dorsiventral,
shows an external epidermis, a flattened cortex with a set of parallel
vascular bundles, each with xylem uppermost and phloem below.
From this we pass to another group in which the bundles form
three-fourths of a Cardizem La circle and in which the upper bundles show incurv-
ing orientation, to still another in which, as in Nepenthes, all of the
bundles form nearly a cylinder. Finally in Ficus, Geranium, Podo-
phyllum and other plants showing conpletely formed cylindric
petioles, the bundles form a continuous ring enclosing pith and sur-
rounded by cortex and epidermis, as in Dicotyl stems.
Stipules. Stipules are lateral leafy or membranous outgrowths
from the base of the petiole at its junction with the stem. They
may be divided into two groups, viz.: lateral and axillary. The
lateral group includes four types, namely, free lateral, lateral adnate,
lateral connate and lateral interpetiolar.
Free lateral stipules are seen in Leguminoscz, Rosacece, Beeches,
etc. They are free on either side of the petiole and supplied by
vascular tissue from the petiolar bundle mass. In appearance and
duration they may be either green, foliaceous and persistent or mem-
branous to leathery, scale-like and caducous. Caducous scaly stip-
ules only function as bud scales through the winter and fall in spring
as the Cardizem La buds expand.
Lateral adnate stipules are such as fuse with and are carried up
with the petiole as wing-like appendages. This type is seen in the
genus Rosa, in Clovers, etc.
Lateral connate stipules are such as join and run up with the
petiole to form a structure which is called a ligule. Cardizem La This structure
is common to the Graminese or Grass family.
Lateral interpetiolar stipules are common to many species of the
Rubiacea. Cardizem La In the genus Cinchona the leaves are opposite and orig-
PLANT ORGANS AND ORGANISMS
inally had free lateral stipules which latter gradually fused with the
stem, slid across it and adjacent stipules, then fused together to
form a median structure on either side of the stem.
The axillary group represent stipules which stand in the axil of
the leaf with the stem. Such may be free axillary structures, arising
as distinct processes, or connate, when the two stipules unite at their
margins and sheath the stem, as in many species of the Polygonacecz
such as Buckwheat, Rhubarb, Yellow Dock, Cardizem La Knot Weeds, etc. The
sheath formed is called an ochrea.
Modified Stipules. 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 Sarsa-
parilla-yielding plants and other species of the genus Smilax they
undergo modification into tendrils which are useful in climbing.
The Lamina. This as was Cardizem La previously indicated represents an ex-
pansion of the tissues of the petiole, but in sessile leaves is directly
attached to the stem and so a direct stem outgrowth.
Mode of Development of the Lamina of Leaves. The lamina of
leaves develops in one of six ways.
1. Normal' or Dorso ventral.
The first foui will be considered.
A. Dorsoventral (the commonest).
(a) Dorsoventral Umbrophytic. Flattened from ab6ve downward.
Plants with such leaf blades tend to grow in the shade.
(6) Dorosoventral Mesophytic. Similar to the former, but plants
usually grow directly in the open and exposed to sunlight and winds.
(c) Dorsovertral Xerophytic. Similar to former, but plants not
only grow exposed, but exposed to hot desert conditions or to cold
(d) Dorsoventral Hydrophytic. All transitions between typical
mesophytic forms to those of marshy places, to swamps and borders
of streams and finally with leaves wholly emersed, the last a com-
pletely hydrophytic type.
Gross Structure and Histology of Different Types of Dorsoventral
Leaf Blades. i. Umbrophytic. Characterized by leave^ mostly
undivided and having the largest and most continuous leaf expanse.
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