The Ovule and Female Gametophyte of Dioon

Abstract

ONLY two of the nine genera of cycads have received any considerable attention from morphologists. In these two, Cycas and Zamia, the life history is fairly well known and many of its phases have been studied in great detail. In Stangeria the development of the sporangia has been investigated, and work on the remaining six genera is fragmentary. Through the courtesy of the Botanical Society of America' the writer was enabled to visit the Mexican tropics for the purpose of securing material of Dioon and Ceratozamia. The hearty cooperation of Governor TEODORO A. DEHESA, who is an active educator as well as a statesman, made it possible to collect an abundance of material in a very short time. Mr. ALEXANDER M. GAW, of the State Bureau of Information, Xalapa, Mexico, had many collections of material brought into Xalapa from the field and forwarded to me after my return to Chicago. To both of these gentlemen I wish to express my sincere thanks, since the investigation would have been very limited without their efficient assistance. The photographs used were made by Dr. W. J. G. LAND. The subject will be treated under the following heads: I. Dioon in the field; II. Material and methods; III. The ovule; IV. The female gametophyte; V. The microsporangium; VI. I The grant was made in December I904 by the Botanical Society, at the St. Louis meeting of the A. A. A. S., and was for the purpose of securing material for a morphological study of Dioon and Ceratozamia.

Development of cycad ovules and seeds. 1. Implication of the ER in primary cellularisation of the megagametophyte in Encephalartos natalensis Dyer and Verdoorn

“…The two principal storage reserves in the megagametophyte of E. natalensis were thus confirmed to be starch and protein, with lipid contributing a minor component. While the megagametophyte tissue of Stangeria eriopus (Lang 1900), Dioon edule (Chamberlain 1906) and D. spinulosum (Dorety 1919) was also seen to have starch in large quantities, the low levels of lipid observed in the present investigation were surprising as gymnosperm species are said to have characteristically fat-storing seeds (Dodd et al 1989). For example, according to Tillman-Sutela et al (1995), the mature megagametophyte cells of species of conifer store significant amounts of lipid.…”

“…2a, b). Newly-formed cells of the megagametophyte of Dioon (Chamberlain 1906) and Ginkgo biloba (Carothers 1907) were reported to be filled with transparent ''cell sap'' by those authors, while in Taxus cuspidata (Sterling 1948), they were said to possess large vacuoles and a lightly staining cytoplasm (i.e. cytomatrix).…”

“…Early wall formation during the transition from the freenuclear (coenocytic) to the cellular megagametophyte of cycads was not recorded for Dioon edule (Chamberlain 1906); Zamia floridana (Smith 1910); Encephalartos spp. (Sedgwick 1924) and Microcycas calocoma (Reynolds 1924).…”

“…Most of the descriptive work on the cycad megagametophyte was done in the early 1900s (Lang 1900;Chamberlain 1906;Smith 1910;Reynolds 1924;Sedgwick 1924). This was before the advent of modern electron microscopes, thus many aspects were unavoidably not able to be resolved.…”

“…The number of free nuclei is reportedly characteristic for different species: e.g. Chamberlain (1906) recorded approximately 1,000 nuclei for Dioon edule while Favre-Duchartre (1958) reported more than 8,000 nuclei for Ginkgo biloba.…”