Journal of Morphology and Anatomy

Fossil and Living Cycads Say "No More Megasporophylls"

Abstract

Xin Wang

The origins of angiosperms and cycads are still mysterious. To understand the evolution of these groups as well as other gymnosperms it was impossible without mentioning a frequently used term “megasporophyll”. “Megasporophyll” is a concept that has been used widely in botany. This term is more or less related with the famous saying “Alles ist Blatt” by Goethe. This term became popular since Arber and Parkin hypothesized that the carpels in the Magnoliales were equivalent to and derived from former foliar parts bearing ovules along their margins (“megasporophyll”). Many botanists uncritically called the parts in all the reproductive organs of seed plants as “sporophylls”, no matter what they actually saw in the plants. However, the fact is that none of the reproductive parts (fossil or living), except those in the Cycadales, are foliar or leaf-like. The female parts in (fossil and extant) Ginkgoales, Coniferales, and Gnetales apparently have nothing to do with any foliar parts, as proven by previous studies. Among the living gymnosperms, Cycadales usually are taken as the most primitive and ancestral, therefore understanding the reproductive organs and their evolution in Cycadales will not only enhance our understanding of Cycadales, but also is hinged with our understanding of all seed plants in general. Interestingly, the female parts of Cycadales are the ones that demonstrate by far the greatest resemblance to leaves, at least in appearance. Thus whether the female parts of Cycadales are truly foliar is a key but rarely asked question hinged with the validity of the term “megasporophyll” and the systematics of seed plants. To verify the validity of this term and its implications, we examined the morphology of both fossil and extant cycads. Our fossil evidence includes the earliest unequivocal fossil of cycad reproductive organ, Crossozamia chinensis (Zhu and Du) Gao and Thomas, recovered from the Permian of China. Unlike seen in living cycads, the ovules in this fossil reproductive organ are apparently inserted on the adaxial rather than strictly the laterals of the female parts. Such an arrangement is not expected for a typical leaf, but rather demonstrates certain resemblance to the sporangia arrangement in Archaeopteris. Parallel to and agreeing with this, the ovules in living cycads, Cycas taitungensis Shen, Hill, Tsou and Chen, have their micropyles oriented to the adaxial rather than the laterals of the parts. Taking into consideration of previous experiment proving that the leaf-like appearance of Cycas “megasporophylls” is due to mechanical pressure and unexpected occurrence of amphicribral vascular bundles with secondary growth in the so-called “megasporophylls” of cycads, we conclude that the female parts of these Cycadales are actually branches bearing ovules. This conclusion rejects foliar nature of female parts in cycads and undermines the validity of the term “megasporophylls”, although the latter has been the most-widely accepted misnomer in botany. The by-product of eliminating this term is that the origins of Cycadales and angiosperms as well as homology of carpels become much easier than assumed before. Considering the long time the term “megasporophyll” inflicting botany and misleading botanists, the influence of eliminating this misnomer in botany cannot be exaggerated. It is not restricted to a single term and its usage, but permeates into all branches of botany, especially the systematics of Cycadales, Angiosperms, and other seed plants.