Pharmaceutical botany - Lighthouse Survival Blog

HISTOLOGY OF ANNUAL DICOTYL STEM 23
Scaly bulbs have narrow imbricated scales, the outer ones not en-
closing the inner. Ex. : Lily.
Tltbers and corms are annual. Bulbs and Rhizomes are perennial.
Exogenous and Endogenous Stems.— Exogenous stems are typical
of Gymnosperms and Dicotyledons and can increase materially in thick-
ness due to presence of a cambium. Such stems show differentiation
into an outer or cortical region and an inner or central cylinder region.
Endogenous stems are _
typical of Monocotyledons
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rially in thickness due to absence
of cambium. Such
stems show no' differentia-
tion into cortical and central
regions.
Histology of Annual
Dicotyl Stem.— (In both an-
nual and perennial dicotyl-
edonous stems endodermis
and pericambium are rarely
seen since each has become
so similar to cortex through
passage of food, etc.)
1. Epidermis, cutinized,
with hairs.
2. Cortex composed of
three zones : an outer or exo-
cortex, whose cells are thin
walled and contain chloro-
plasts; a middle or medio-
cortex, consisting of cells of
indurated walls giving extreme pliability and strength, an inner or endo-
cortex, a very broad zone of thin and thicked-walled parenchyme cells.
3. The innermost layer of cells of the cortex called the endodermis.
(Not generally distinguishable.)
4. Pericambium. (Not generally distinguishable.)
5. Fibrovascular bundles of open collateral type arranged in a
circle with primary medullary rays between the bundles.
6. Pith.
Fig. 17.—Photomicrograph of cross-section
of stein of Aristolochia sipho, where cambial
activity is just beginning, a, Epidermis; b,
collenchyma; c, thin-walled parenchyma of the
cortex, the innermost cell layer of which is the
starch sheath or endodermis; d, sclerenchyma
ring of the pericycle; e, thin-walled parenchyma
of the pericycle; /, primary medullary ray; g,
phloem; h, xylem; i, interfascicular cambium; j,
medulla or pith. X20. {From Stevens.)