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Cycadeoidea Saucer Shaped Structure

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Cycadeoidea Saucer Shaped Structure 1 | P a g e —The bracts open up at maturity to form a broad Cycadeoidea saucer shaped structure. Systematic Position —There are about 20 pinnate microsporophylls Class: Cycadopsida arranged in a whorl at the base of ovuliferous receptacle. Order: Cycadeoidales/Bennettitales —Two rows of kidney shaped synangia are borne on Family: Cycadeoidaceae the inner surface of each pinnule of a Genus: Cycadeoidea microsporophyll. Microsporophylls were united at the base and free above which is comparable to the segments of an orange. Distribution: Cycadeoidea is the only genus under — Each synangium bore 20-30 tubular pollen sacs the family Cycadeoidaceae and has a wide or sporangia containing monocolpate (monosulcate) geographical distribution, especially in North pollen grains. America, Europe and India during Upper Jurassic to — The synangium consisted of palisade like cells and Upper Cretaceous. dehisced by means of an apical slit into two equal halves. External Features: — Numerous tiny stalked orthotropous ovules are —The plants have ovoid or short columnar which present in group at the conical or dome shaped apex were unbranched or apparently branched. The of the fertile shoot. They are interspersed with trunks were massive, not more than 1 m in length interseminal scales. Their number is equal to the and 60 cm in diameter. number of ovules. Their heads are enlarged into a —The surface of the stem is covered with prominent club which are fused with the adjoining interseminal rhomboidal leaf bases and multicellular hairs in scales in such a manner that it forms a continuous between them. surface layer with openings. Through the openings — The trunk had spirally arranged leaf bases micropyles project. The layer forms an external embedded in ramenta of flat, tongue shaped scales. protective covering over the developing seeds. — The leaves have never been found attached to —Each ovule has two layered integument, nucellus the stem. The leaves were borne spirally in a crown with beak and a prominent pollen chamber. at the top of the trunk. The nucellus is composed of thin walled cells. The — The flowers were borne in the axil of leaf bases cells close to chalaza are smaller than the cells and developed simultaneously. Thus, the plants which are towards micropyle. were monocarpic i.e., they flowered once during — The seeds represent small ovular structure with their lifetime. dicotyledonous embryo. —Linear tetrad of megaspores has been reported. Anatomy: —A nucellar plug has been reported also. Stem: —The nucellus is fused with integument except at the —The trunk shows a large parenchymatous pith apices. The integument does not receive any surrounded by a thin zone of primary wood with vascular supply. endarch protoxylem and secondary wood with —The ovule is surrounded by five to six interseminal scalariform tracheids and small wide rays. The scales, each with its own vasculature. wood is manoxylic wood. —Cortex: Parenchymatous interspersed with gum Relationship with other groups: canals and leaf traces. Relation to Cycadales —Leaf traces: The leaf traces traversing the cortex Both the cycads and cycadeoids have common are C-shaped and pass directly through the ancestry, so, they have a number of common secondary xylem to the cortex where they again characteristics. These are: divide and arrange themselves in a horse-shoe- 1. The plants are small trees with barrel- shaped fashion without girdling. shaped trunk. Pinnules: 2. The stem anatomy shows a broad cortex, —Distinct upper and lower epidermis, epidermal large pith and relatively thin vasculature cells are thick walled showing siphonostele with endarch xylem —Mesophyll: differentiated into palisade and spongy 3. The wood is manoxylic. parenchyma 4. Compound leaves show similarity in form, Vascular bundle: conjoint, collateral, the size and venation. — phloem towards lower epidermis and 5. Presence of orthotropus ovule. xylem toward upper epidermis 6. The presence of dicotyledonous embryo. — Prominent bundle sheath 7. The presence of monocolpate (monosulcate) Reproduction: pollen. —Cycadeoid flowers are bisexual, borne on a short Major difference between Cycadeoidales and pedicel and are protected by about 100 spirally Cycadales are as follows: arranged pinnate bracts which are hairy. They are —Haplocheilic stomata in Cycadales whereas covered by a ramentum of scales. Cycadeoidales have syndetocheilic stomata, ANC, Patna/Dept of Botany/ BSc II(Hons)/ Sushil 2 | P a g e — 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. 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. Add diagrams ANC, Patna/Dept of Botany/ BSc II(Hons)/ Sushil .
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