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In this article we will discuss about the affinities of Cycadeoids which belong to the family of Class Cycadopsida.
A. Relation to Ferns:
Cycadeoids resemble ferns in the following characteristics:
1. The abundance of multicellular rementa on the plant body.
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2. The presence of scalariform tracheids, large pith and direct leaf traces.
3. The pinnatified microsporophyll with lateral rows of synangia in cycadeiods are comparable to the sori of the Marattiales.
B. Relation to Pteridospermales:
Cycadeoids probably originated from pteridosperms, thus they share many common features. These include:
1. The presence of barrel-shaped trunk.
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2. The presence of ramental hairs.
3. The presence of direct leaf traces.
4. The stem shows large pith and cortex with manoxylic wood.
5. The presence of leafy microsporophyll bearing synangia.
6. The bisporangiate ‘flower’ of Cycadeoidales may be related to bisporangiate fronds of Cycadofilicales.
C. Relation to Cycadales:
Both the cycads and cycadeoids have their common ancestry, thus these groups share many common characteristics.
These are:
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1. The plants are small trees with barrel-shaped trunk.
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2. The stem anatomy shows a broad cortex, large pith and relatively thin vasculature showing siphonostele with exarch xylem.
3. The wood is manoxylic.
4. Compound leaves show similarity in form, size and venation.
5. The presence of dicotyledonous embryo.
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6. The presence of monocolpate (monosulcate) pollen.
D. Relation to Angiosperms:
Many botanists like Arber and Parkin, Bessey, Hallier, Hutchinson, believed that the Cycadeoidales is the ancestor of angiosperms. This view is based on the similarity in cycadeoid strobili and the flowers of some of the more primitive angiosperms such as Mangoliales and Ranales. There are many other characteristics noted in cycadeoids that closely resemble the primitive angiosperms.
These are:
1. The flowers of Magnoliales are showy with extended floral axis and numerous floral parts are arranged spirally on the floral axis. This could be compared to the strobilus of cycadeiods, where conical receptacle represents floral axis on which ovules and microsporophylls are spirally arranged.
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2. The spirally arranged hairy bracts of cycadeoid are comparable to the perianth of angiosperms.
3. The syndetocheilic stomata of cycadeoids are comparable to the mesoparasitic stomata of Magnoliales.
4. The endarch siphonostelic vasculatures with scalariform tracheids of Cycadeoidales are the typical characteristics of sympetalous angiosperms.
5. The dicotyledonous embryos are present in both the cycadeoids and Magnoliales.
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Pant and Kidwai (1977) disagreed over the above view. Based on the geological occurrence of cycadeoids and angiosperms, they argued that cycadeoid flower represents a parallel line of evolution to angiosperms.
It has been assumed that the homologies between cycadeoids and angiosperms are due to a long course of evolution. The cycadeiods have naked seeds (typical of gymnosperms) with an admixture of sterile interseminal scales. Moreover, wood rays are devoid of marginal cell which is typical of angiosperms.
