CYC'ADATEZE (Neo-Lat. nom. pl., from Neo-Lat. (Teas. Gk. k.i.Nor„ kykas, African cocoa palm). One of the four living groups of gym nosperms. In the present flora nine genera of cycads are recognized, which contain about eighty species. They are exclusively tropical, and are about equally distributed between the eastern and western tropics. The stems are either col umnar shafts, crowned with a rosette of huge fern-like foliage leaves, having the general habit of tree-ferns and palms; or they are like great tubers, completely invested by an armor of thick leaf-bases and scale leaves, and crowned with fern-like leaves as in the other ease. The group is of especial interest on account of its fern-like characters, and there. seems to be general con sent that it is a group which has been derived from the ferns. Although fern-like in appear ance and in many structures, the cycads pro duce seeds and mast be associated with seed plants, and since the seeds are exposed, they are gymnosperms.
A discovery recently made emphasizes strik ingly their fern-like character. In fertilization the male cells bear many cilia and are free swimming, while in all other known seed-plants they have no power of motion. This retention of the old ciliated sperm habit by undoubted seed-plants is a very interesting transition con dition. The group is also of interest in showing that the pollen-tube, which is connected in other seed-plants with the transfer of the male cells to the egg, may not have been originally devel oped for this purpose. In the eyeads the tube is developed by the pollen-grain. but it branches freely through the ovule and acts as an absorbing system, the male cells never entering it. It was probably later in the evolution of plants that this absorbing system came to be used as a means of transferring the male cells. Another charac teristic feature of the group is that the seed coat, instead of being entirely hard, as in the conifers, is plum-like, since it develops in two layers. the inner hard and bony, the outer pulpy, making the ripe fruit resemble a plum.
The structure of the stein in many cases is not essentially different from that of the ordinary conifer-stems, but in certain genera it presents unusual and suggestive features. In these cases the primary cambium, by means of which the ordinary increase in diameter is effected, is short-lived, and a series of secondary cambiums is organized in the cortex. The vascular bundles thus formed in the cortex are frequently of the concentric type. which is characteristic of the ferns and not of the seed-plants.
Although the strobili of the eyeads have the general structure of those of conifers, in some cases the spore-leaves do not resemble those of tlie other seed-plants. For example, in the genus Cveas there is no distinct stamen in the ordinary sense, but a leaf-like body whose under surface is thickly covered with groups of sporangia, as in ordinary ferns.
The geographic distribution of eyeads is as follows: ln the Oriental tropics the genera are: Cycas, containing about sixteen species and rang ing throughout tropical Asia, the East. Indies, and the Australasian region; Macrozamia. with fourteen species, and the monotypie Bowenia, both strictly Anstralasian; Encephalartos, with twelve species, and the monotypic Staugeria. both restricted to Africa. In the Occidental tropics the largest is Zamia. with about thirty species, ranging throughout tropical and sub tropical America; Ceratozamia. with six species. and Dioon. with one species. are American; while the monotypic Microcyeas belongs to Cuba. In general it is true that Cycas and Zinnia are the typical cyeads of the two hemispheres, while the other genera represent relatively isolated forms which bear the stamp of local condition:.
Fossil, Foams.. The Cycadaceac arc a group of very great antiquity, the genus Cycas itself ap pearing to be the oldest of all the genera of the family, for it is known from the Carboniferous limestone. This singular type has, since that early time, persisted in modern vegetation, where it stands without any near relatives. The remains of attain their greatest de •velopment in the 3lesozoic formations, especially in the Jurassic rocks ( Purbeck and Wealden) of England and France, and in the Eimer Creta ceous of the Black Hills of North America, in which latter formation the silicified short cylin drical stems with their closely crowded, spirally disposed leaf-bases are quite eharaeteristie fos sils. Also the leaves and fructification of the Cycadacem are of frequent occurrence in the Mesozoic beds of the Northern Hemisphere, show ing that these plants flourished there in that era. In the Tertiary they are rare in Europe, though still occurring in southern Europe. Later their last survivors withdrew- mostly to the Southern Hemisphere. With the fossil Cyea dacem are usually mentioned the Medu•lasfe, remains of stems from the Carboniferous and Per mian rocks, which show many points of resem blance to the Cycailace:e, while departing from them in some important features. Consult: Von Sohns-Laubaeh, Fossil Botany (Oxford, 1891) ; Ward. "Description of the Species of Cycadeoi dea or Fossil Cycatlean Trunks from the Lower Cretaceous Rim of the Black Hills," Proceedings of the United States national Museum, vol. xxi. (Washington, 1899) ; id., "Description of a New Genus and Twenty New Species of Fossil Cycadean Trunks from the Jurassic of Wyoming." in Proceedings of the Washington Academy of Science, vol. i. (Washington, 1900).