Presented by

Cycadeoidea �

(Fossil Gymnosperms)

Dr.B.Adinarayana

Department of Botany

GDC Rajampeta

Cycadeoidea �

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Systematic Position

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Class: Cycadopsida

Order: Cycadeoidales/Bennettitales

Family: Cycadeoidaceae

Genus: Cycadeoidea

� Distribution: Cycadeoidea is the only genus under the family Cycadeoidaceae and has a wide geographical distribution, especially in North America, Europe and India during Upper Jurassic to Upper Cretaceous.

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External Features:

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—The plants have ovoid or short columnar which were unbranched or apparently branched. The trunks were massive, not more than 1 m in length and 60 cm in diameter.

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—The surface of the stem is covered with prominent rhomboidal leaf bases and multicellular hairs in between them.

— The trunk had spirally arranged leaf bases embedded in ramenta of flat, tongue shaped scales.

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— The leaves have never been found attached to the stem. The leaves were borne spirally in a crown at the top of the trunk.

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— The flowers were borne in the axil of leaf bases and developed simultaneously. Thus, the plants were monocarpic i.e., they flowered once during their lifetime.

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Anatomy:

Stem:

• The trunk shows a large parenchymatous pith surrounded by a thin zone of primary wood with endarch protoxylem and secondary wood with scalariform tracheids and small wide rays. The wood is manoxylic wood.
• Cortex: Parenchymatous interspersed with gum canals and leaf traces.
• Leaf traces: The leaf traces traversing the cortex are C-shaped and pass directly through the secondary xylem to the cortex where they again divide and arrange themselves in a horse-shoe-shaped fashion without girdling.

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Pinnules:

Distinct upper and lower epidermis, epidermal cells are thick walled

Mesophyll: differentiated into palisade and spongy parenchyma

Vascular bundle: conjoint, collateral, phloem towards lower epidermis and xylem toward upper epidermis. Prominent bundle sheath

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Reproduction:

Cycadeoid flowers are bisexual, borne on a short pedicel and are protected by about 100 spirally arranged pinnate bracts which are hairy. They are covered by a ramentum of scales.

—The bracts open up at maturity to form a broad saucer shaped structure.

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—There are about 20 pinnate microsporophylls arranged in a whorl at the base of ovuliferous receptacle.

—Two rows of kidney shaped synangia are borne on the inner surface of each pinnule of a microsporophyll. Microsporophylls were united at the base and free above which is comparable to the segments of an orange.

— Each synangium bore 20-30 tubular pollen sacs or sporangia containing monocolpate (monosulcate) pollen grains.

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— The synangium consisted of palisade like cells and dehisced by means of an apical slit into two equal halves.

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— Numerous tiny stalked orthotropous ovules are present in group at the conical or dome shaped apex of the fertile shoot. They are interspersed with interseminal scales. Their number is equal to the number of ovules. Their heads are enlarged into a club which are fused with the adjoining interseminal scales in such a manner that it forms a continuous surface layer with openings. Through the openings micropyles project. The layer forms an external protective covering over the developing seeds.

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• —Each ovule has two layered integument, nucellus with beak and a prominent pollen chamber.
• The nucellus is composed of thin walled cells. The cells close to chalaza are smaller than the cells which are towards micropyle.
• — The seeds represent small ovular structure with dicotyledonous embryo.
• —Linear tetrad of megaspores has been reported.
• —A nucellar plug has been reported also.
• —The nucellus is fused with integument except at the apices. The integument does not receive any vascular supply.
• —The ovule is surrounded by five to six interseminal scales, each with its own vasculature.

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Relationship with other groups:

Relation to Cycadales

• Both the cycads and cycadeoids have common ancestry, so, they have a number of common characteristics. These are:

1. The plants are small trees with barrel-shaped trunk.

2. The stem anatomy shows a broad cortex, large pith and relatively thin

vasculature showing siphonostele with endarch xylem

3. The wood is manoxylic.

4. Compound leaves show similarity in form, the size and venation.

5. Presence of orthotropus ovule.

6. The presence of dicotyledonous embryo.

7. The presence of monocolpate (monosulcate) pollen.

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Major difference between Cycadeoidales and Cycadales are as follows:

—Haplocheilic stomata in Cycadales whereas Cycadeoidales have

syndetocheilic stomata,

— Girdling leaf traces are absent in Cycadeoidales whereas they

are present in Cycadales.

—Arrangement of microsporophylls is whorled in Cycadeoidales

whereas it is Spiral in Cycadales.

— Cones are bisporangiate in Cycadeoidales whereas it is

monosporangiate in Cycadales.

—Arrangement of stalked ovules along with interseminal scales is

unique feature of Cycadeoidales which is not found in

Cycadales.

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On the basis of such differences Andrews opined that both groups have evolved along two different and independent lines.

Relation to Pteridospermales

• Cycadeoids probably originated from pteridosperms, thus they share many common features. These include:
• 1. The presence of barrel-shaped trunk.
• 2. The presence of ramental hairs.
• 3. The presence of direct leaf traces.
• 4. Presence of syndetocheilic stomata
• 5. The stem shows large pith and cortex with manoxylic

wood.

• 6. The presence of leafy microsporophyll bearing synangia.
• 7. Presence of cupule
• Acc. to some authors they are two lines of evolution from Cycadofilicales-one gave rise to monosporangiate forms like cycads the line gave rise to Cycadeoids having both mono- and bisporangiate forms. The major difference is in the organization of ovule as they are smaller and stalked in Cycadeoidales.

Relation to Angiosperms

• Many botanists like Arber and Parkin, Bessey, Hallier, Hutchinson believed that Cycadeoidales is the ancestor of angiosperms. This view is based on the similarity between cycadeoids and angiosperms.
• The similarity is due to strobili and the flowers of some of the more primitive angiosperms such as Magnoliales and Ranales. There are many other characteristics noted in cycadeoids that closely resemble primitive angiosperms. These are:
• 1. The flowers of Magnoliales are showy with extended floral axis and numerous floral parts are arranged spirally.
• 2. The spirally arranged hairy bracts of cycadeiods are comparable to the perianth of angiosperms.
• 3. The syndetocheilic stomata of cycadeiods are comparable to the mesoparacytic stomata of Magnoliales.

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