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Cordaitales
The Cordaitales represent the most
ancient order of the class Coniferopsida,
which appeared in the Devonian. They
were the only representatives of the
Coniferopsida in that age. They were at
their peak of development during the
carboniferous period of the Palaeozoic era
and were succeeded by the Ginkgoales
and the Coniferales in the Mesozoic era.
Classification
The order has been divided into three
families:
(i)
Cordaitaceae,
(ii)
Eristophytaceae and
(iii)
Poroxylaceae.
The order is represented in India by 4
genera and 26 species.
CORDAITACEAE
External characters
—The members of this family formed the
world's first great forests towards the
close of Lower Carboniferous.
—It included tall trees (upto at least 30
meters in height) that grew in dense
stands.
—The stems attained a diameter of 65
centimeters or a little more.
—The branching was lateral and confined
only to the top of the tree.
—The leaves appeared on the branches.
They were— spirally arranged,
petiolate,
simple subulate,
spathulate or grass like with entire
margins and obtuse or acute apices.
The
venation
was
parallel.
—The
leaves
showed
considerable
variation among the members of the
family and were put under the type genus
Cordaites. This name is now applied to
the stem as well as to the whole plant.
—Based on the differences among the
leaves of various Cordaitaceae the genus
Cordaites has been sub-divided into three
sub-genera:
1. Eu-Cordaites
2. Dory-Cordaites
3. Poa Cordaites
—The
roots were named Ameylon. They
arose from the base of the stem and were
repeatedly branched.
Dept of Bot/M.Sc. Sem-I (Bot)/ANC, Patna/Sushil
ANATOMY
Stem
—The
cordaitalean stems are Mesoxylon,
Cordaites,
Metacordaites,
Parapitys,
Coenoxylon, Mesopitys, Cordaicladus and
Artisia.
—In
Cordaites the stem had a large cortex
with sclerenchymatous patches scattered
amid the parenchyma. The secretory sacs
were also present. The pith was wide and
extensive and peculiar in being discoid.
The characteristic feature of the pith is
the presence of large air chambers
separated by diaphragms. The pith was
transversely septate at the margins. The
petrified
material
indicated
that
diaphragm was incomplete.
—The
primary xylem forms a narrow zone
around the pith and consists of large
number
of
endarch
protoxylem.
—In Mesoxylon the xylem in mesarch.
—The
stem genus Metacordaites had no
air chambers on the pith and the xylem
was endarch, whereas in Parapitys the
xylem was mesarch.
—In
Coenoxylon and Mesopitys the pith
had thick walled or sclerenchymatous
patches distributed in the pith. Both has
endarch primary xylem. The bundles
were open.
—As
a result of secondary growth
compact or pycnoxylic secondary wood is
formed, that is, usually traversed by
narrow or uniseriate vascular rays.
—The
tracheids were long and narrow.
The vascular pitting was primarily
restricted to the radial walls. The
bordered pits were hexagonal in outline
and occurred usually in uniseriate rows.
Metaxylem also showed some scalariform
pitting. It is believed that in Mesoxylon
the leaf traces were mesarch, but most of
the cauline bundles had endarch xylem.
—The
phloem is not well preserved. It
showed
sieve
tubes
and
phloem
parenchyma. Presence of phloem fibers is
doubtful.
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The leaf
The cordaitean leaf traces were either
single or double at their point of origin
from the primary vasculature. In
Cordaites the strands were endarch
whereas in Mesoxylon they were
mesarch.
Root
—The
roots in Cordiatales (Ameylon) were
usually triarch, exarch and protostelic. —
—presence of
periderm around the
secondary cylinder.
—Extensive secondary wood
—Another root genus is Premnoxylon. It
was
—exarch with the number of xylem
poles varying between two to seven or
even greater
—pith
had
sclerenchymatous
patches and the
—secondary
xylem formed a
distinct ring
—The root cortex was divisible into
an outer and an inner cortex.
—Periderm present in the outer
cortex.
Leaves:
The
leaves
showed
xerophytic
adaptations.
—distinct epidermis on the upper and
lower surfaces (bifacial). The epidermal
cells were thick walled.
—The
sclerenchymatous
hypodermis
occurred in the form of distinct patches
on both sides in C. angulostriatus
whereas in C. rhobinervis it formed a
continuous layer interrupted by distinct
sclerenchymatous
bundle
sheath
extensions.
—Clear distinction into palisade and
spongy parenchyma in C. lingulatus.
—Vascular Bundles:
Mesarch
The bundles were surrounded by a
distinct
sclerenchymatous
bundles
sheath.
—The
stomata were arranged in distinct
bands along the entire length of the leaf.
—The guard cells were surrounded by 4-6
subsidiary cells.
—Two polar subsidiary cells in every
stoma abut on the ends of the guard cells.
Dept of Bot/M.Sc. Sem-I (Bot)/ANC, Patna/Sushil
Reproductive organs
—The
fructification of Cordaites is
Cordatianthus (Cordaianthus).
—The fructifications occur among the
leaves on the stem.
— Each fructification was borne on a
vegetative shoot and consisted of an axis
which bore spirally arranged bracts with
axillary
reproductive
shoots/dwarf
shoots/short shoots.
Male
strobilus/microsporangiate
short shoot:
—The three well studied forms include
Cordaitanthus concinnus, C. penjonii and
C. saportanus.
—Each dwarf shoot had a central axis with
a large number of spirally arranged and
uni-nerved scales.
—The scales at the proximal end- sterile
whereas those near the apex or the distal
ones-fertile
—The scale possessed 4 (C. saportanus)
or 6 (C. penjonii) or many microsporangia
or pollen sacs.
—Microsporangia:
—
elongate and finger like
structures
—each
microsporangium
separately
vascularized.
—The vascular strand that supplied the
sporophylls were mesarch and forked
dichotomously near their apices to send a
branch
to
the
base
of
each
microsporangium.
—The microsporangia had a single layered
wall enclosing large number of pollen
grains or the microspores(65  to 150 .
in diameter).
—The microspores had an equatorial
bladder or an air cavity enclosed by the
exine. This is the saccus.
—The inner wall or the intine enclosed a
central cavity which contained cellular
remains of microgametophytes.
—The
cavity
of
the
germinated
microspore is occupied by a row of
peripheral male prothallus cells and a row
of central spermatogenous cells.
—Each spermatogenous cell divided into
two
spermatozoids
that
were
multiciliated.
— These were liberated into archegonial
chamber by the disintegration of the
intervening nucellar tissue.
—Pollen tubes have not been found.
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—The
pollen grains were trilete on the
proximal surface.
Female strobilus/megasporangiaqte
strobilus or dwarf shoot:
—The organization of the female stroblus
is similar to that microsporangiate dwarf
shoot.
— They arise as lateral outgrowths from
the inflorescence axis..
— Each strobilus had a central axis with
spirally arranged appendages.
—The basal or proximal appendages were
sterile whereas the distal ones were
fertile and bore one to three ovules
terminally.
—In Cordaitanthus pseudofluitans the
distal
fertile appendages/sporophylls
were
greatly
elongated.
They
dichotomized one or more times and bore
two to more terminal ovules.
—Ovules were more or less pendulous,
fattened and showed a tendency to be
recurved towards the main axis.
—The fertile appendages later grow more
and project beyond the short shoot.
—In C. zeilleri the dwarf shoot bore only
four fertile appendages or sometimes
only one. The megasporophylls were
unbranched and bore only one terminal
erect ovule.
—The ovules were bilaterally symmetrical
and were enclosed with in a single, but
two parted integument that were
completely free from the nucellus.
— A distinct pollen chamber was found
containing germinated pollen grains. The
ovules appeared heart shaped from the
broad side.
—The single median bundles entered the
sporophyll and divided into three just
below the ovule.
—The integument consists of two or three
distinct layers of tissues (i) sarcotesta or
the outer fleshy layer; (ii) sclerotesta or
the middle stony layer; and (iii)
endotesta or the inner narrow and fleshy
layer.
In Cardiocarpus spinatus the sarcotesta is
fleshy, whereas the sclerotesta is hard
and
stony and is highly resistant to
decay.
— The nucellus, in the mature ovules,
appears as a thin band
of tissue
surrounding the female gametophyte.
Dept of Bot/M.Sc. Sem-I (Bot)/ANC, Patna/Sushil
— The
female gametophyte consists of a
parenchymatous mass of cells with two
archegonia towards the micropylar end.
—The
female gametophyte in most of the
ovules encroaches upon the nucellus
almost to the integument and also
develops a characteristics endosperm
beak which supports the nucellar beak
and is called the tent pole as in Ginkgo
biloba.
—The archegonial neck opens into a
distinct archegonial chamber which
surrounds the tent pole.
POROXYLACEAE
This family is represented by a single
genus Poroxylon whose seeds are put
under the form genus Rhabdospermum.
ERISTOPHYTACEAE
It includes the form genera Eristophyton,
Endoxylon and Bilingea.