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Journal Article

Morphology of the Trunk and Development of the Microsporangium of Cycads

Frances Grace Smith
Botanical Gazette
Vol. 43, No. 3 (Mar., 1907), pp. 187-204
Stable URL: http://www.jstor.org/stable/2466225
Page Count: 20
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Abstract

1. The stem of Zamia floridana is a sympodium, a vegetative point lying at the base of each strobilus. The staminate strobili develop one after the other from the successive vegetative points, each strobilus with a small circle of leaves, and all enclosed within the larger scale leaves of the first strobilus. 2. The youngest staminate strobili received June 1 showed the sporophylls arising in acropetal succession, by a periclinal division of a hypodermal cell, and later by the divisions of other hypodermal cells and layers beneath them. 3. The microsporangia cover the abaxial face of the sporophylls in Ceratozamia mexicana, and are grouped upon each flank in Zamia. In Dioon, Encephalartos villosus, E. Caffer, and Macrozamia Miquelii there are intergrades, where the sporangia cover the sporophylls of the central part of the strobilus, but are in two groups upon the sporophylls of the tip and base. 4. The microsporangia are grouped in sori (two to six sporangia in each sorus), which are raised on a cushion of tissue into which a vascular bundle passes. 5. In Zamia and Ceratozamia the sporangia of a sorus are sometimes free only half their length. 6. One strobilus of Zamia floridana had two to forty-eight sporangia on its different sporophylls, and some sporophylls in which the middle region was entirely covered. 7. Strobili of Zamia floridana received July 25 showed in tangential sections three or four developing sporangia upon either flank of the sporophyll the youngest upon the margin. 8. The archesporium is a single hypodermal cell which usually divides first anticlinally, followed by periclinal divisions; the outer plate of four cells developing the wall layers, the inner plate the sporogenous tissue. 9. The wall of the mature microsporangium is composed of four to seven layers of cells, the cells of the two layers next the tapetum being tabular. The apex of the sporangium consists of thick-walled cells and beneath it there are isodiametric, thick-walled cells. Extending from the apex on either side of the line of dehiscence there is a band of thicker-walled cells, which suggests the structure of the sporangium of Angiopteris. 10. Beneath the line of dehiscence there is a plate of cells extending toward the center of the sporangium, which contain crystals; and in Zamia these cells seem to degenerate as the sporangium breaks. 11. Stomata occur on the microsporangium, the guard cells being deeply sunken and thick-walled. 12. The tapetum is derived from sporogenous tissue, at least in part, and the sporogenous ceils at the base of the sporangium function as tapetal ceils, which may project into the sporangial chamber. Sometimes, at least, the tapetal nuclei divide mitotically, for spindles were seen; and there are often two nuclei to a cell. The tapetum remains a distinct layer up to the tetrad stage. 13. There is some additional sterilization of sporogenous tissue in the sporangium. 14. The divisions of the spore mother cells conform to the descriptions of JURANYI and TREUB. The reduced number of chromosomes in Ceratozamia and Zamia is twelve. 15. The output of spores per sporangium for Zamia floridana is 500-600, for Ceratozamia mexicana 8,000, and for Encephalartos villosus 26,000. Therefore the output per sporangium in these species increases according to the number of sporangia upon the sporophyll. 16. The pollen grains are three-celled at the time of shedding, the cells being the prothallial, the generative, and the tube.

Notes and References

This item contains 27 references.

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