Bennettitalean Theory

The Bennettitalean theory was first proposed by Saporta and Marion (1885), later on elaborated by Arber and Parkin (1907). Bennettitales is the extinct group of gymnosperms which existed long back in the Mesozoic era. These investigators worked out the similarities between the stromboli of the Bennettitalean plant Cycadeoidea dacotensis and flowers of the primitive angiosperm Magnolia. Both these structures are bisexual, and have an elongated axis having protective bracts, microsporophyll’s and megasporophylls, arranged successively from below upwards.

However, in spite of these superficial resemblances, further studies have shown several differences, which are as follows:

1.In Magnolia, the microsporophyll’s (stamens) are free and are spirally arranged on the axis, whereas in Bennettitales they are whorled and mostly connate.
2. In Bennettitales, the megasporophylls are greatly reduced, simplified stalk-like structures, each bearing a solitary terminal erect ovule. Between megasporophylls, there are sterile scales (inter-seminal scales), which are protective in function. No such structures are present in the flowers of Magnolia.
3. The micropylar tube formed in the ovules of Bennettitales are absent in the angiosperms and the pollen grains are shed on the stigma of the carpel (megasporophyll).

4. The seeds of Magnolia and other primitive angiosperms are with copious endosperm and small embryo while those of Bennettitales are non-endospermic with a large embryo.

5.In the bennettitalean stem there is a large pith, a thin vascular cylinder and a thick cortex, while the angiosperm stem has a small pith, a thick vascular cylinder and a thin cortex.

These differences indicate that Bennettitales cannot be considered as the ancestors of angiosperms. The similarities with angiosperms, most probably, might have resulted due to a common ancestry and parallel evolution. Arber and Parkin have postulated that the two groups did have a common origin from seed ferns and they might have diverged very early.