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Related article: xylem finally fills Biaxin Xl up the patches between
the arms. The patches of bast fibers get flattened out. The
pericambium has a tendency to start division into an inner and
outer layer. The outer layer Biaxin Xl becomes a cork cambium (phellogen)
surrounding the bundle inside of the endodermis. It cuts off cork
tissue Biaxin Xl on its outer face, hence all liquid material is prevented from
filtering through and cortex including endodermis, as well as the
d ; Vc r -L e S
portion of root: lettered as in A;
* C0rk "
PLANT ORGANS AND ORGANISMS 127
epidermis, shrivel and dry up and separate off at the age of two
to three Biaxin Xl months. The cork cambium (phellogen) may lay down
secondary cortex internal to itself and external to the phlcem.
Patches of cells of the inner layer of pericambium divide rapidly
and are called interfascicular cambium. These join the intrafasci-
cular cambium to form a continuous cambium ring which then
cuts off additional secondary xylem on its inner face and secondary
phloem on its outer face pushing inward the first-formed or proto-
xylem and outward the first-formed or protophloem. The medullary
rays become deepened.
Thus, in a transverse section made through a portion of a Dicotyl
root showing secondary growth, the following regions are noted pass-
ing from periphery to center:
2. Cork cambium (phellogen)
3. Secondary cortex
5. Secondary phloem
7. Secondary xylem
Strands of cells extending radially from the cortex to the center
of the section separating each open fibro-vascular bundle from its
neighbors. These are called medullary rays.
Histology and Development of a Dicotyl Root (California Privet)
A. Make a permanent mount of a T. S. of the root of the California
Privet (Ligustrum Calif ornicum) cut just above the root cap, and
note the following structures, passing from periphery toward the
center (see Fig. 61):
1. Epidermis, composed of a layer of epidermal cells whose
outer walls have been infiltrated with a substance called Cutin.
2. Hypodermis, a layer of somewhat thick walled cells just be-
neath the epidermis.
3. Cortex, composed of cortical parenchyme cells with small
angular intercellular air spaces.
4. Endodermis, or innermost layer of cells of the cortex, whose
radial walls are lenticularly thickened.
5. Pericambium, of a layer of actively growing meristematic
cells, which has the power of producing lateral rootlets.
6. Radial nbro-vascular bundle of Biaxin Xl five xylem arms alternating
with as many phloem patches. Note the narrow spiral tracheae in
the xylem patches.
PIG. 61. Photomicrograph of a transverse section of a California Privet root
of primary growth showing epidermis (e) ; hypodermis (k) ; cortex (c) ; endodermis
(en); pericambium (p); a xylem arm of the radial bundle (/) and pith (m).
The section you have just studied illustrated in general the appear-
ance of any Dicotyl root of primary growth.
B. Mount permanently another T. S. cut through the same root a
short distance above the first.
Note that this is somewhat larger in diameter. Observe the root
hairs starting from the epidermis; a broad cortex; a large clear and
open looking endodermis; then pericambium; next, a central patch
of xylem showing a faint pentarch relation. Pushed out are five
PLANT ORGANS AND ORGANISMS
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