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Cycadales

cortex, cycads, cycas, forest, fig, plants, species, leaf and leaves

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CYCADALES, slic-a-clalez (from Cytas Neo Lat. nom. p1. of Gr. ainats; the orig inal name of the African cocoa-palm), a greater group of tropic to sub-tropic naked-seeded or gymnospermous plants, the cycads. The large pith of the thick palm-like trunk of certain species of the genus Cycas is the source of the sago starch of commerce, whence the common name, *sago palm.* The existing forms are only an ornate remnant of an ancient and varied alliance, the nearest living relative being the or maidenhair tree.

At least two distinct Cycadalean types are recognizable; the cycads proper include all extant forras and have only a short fossil record, while the cycadeoids, now wholly extinct, were cosmopolitan throughout all of Mesozoic time. The first group is comprised within the order Cycadacez, to which all cycadaceous types were until recent years supposed to belong. The second group may be arbitrarily brought within the order Bennettitacece, with far the broader relationships and an immensely varied history. But it appears that variants of both groups go back to the Carboniferous.

I. cycads are a primitive megaphyllous and composite type with wood structure like conifers (and Cordaites), certain frond and other characters of ferns, and the outward habit of palms. As in the latter, the stem elongates by the slow growth of a terminal bud, with the unfolding of successive crowns of leaves or fronds spirally arranged in close order, As the leaves wilt down, there is formed from their bases an outer more or less persistent armor, which gives the stem its very character istic appearance. For trunk-forming plants, the cycads are mostly small or even pygmic. They include tuberous to columnar sparse branched forms, and vary in size from under ground trunks a few centimeters in diameter, with fronds no more than a decimeter long, to moderately tall forest forms.

It was once suggested that Cycas (see Fig. 2), in which the so-called carpellary leaves alternate with the foliage leaves in armor for mation, grew taller than forms bearing cones only. But such is not the fact. The Australian Macrozamia Hopei reaches a height of 60 feet, or nearly double that of any Cycas, and the Mexican Dion spinulosum is nearly as tall. Both these are columnar, rather than branched types. The most robust trunks may reach a meter in diameter, and the longest fronds a length of four meters. Several Central Ameri can and Peruvian species are epiphytic. Most of the cycads branch one or more times after reaching a certain age, and all are handsome and easily grown greenhouse plants. The Japanese gardeners display examples of their native species Cycas revohtta, upward of 300 years old, and as freely branched as screw pines. A pot-type is shown in Fig. 3. One of the most interesting of all species is the Cuban Microcy cas calocoma, a slender stemmed forest form about 30 feet high, also branched in a manner recalling the habitus of the screw pines. The

cycads are widely dispersed in tropical and sub tropical regions, though seldom abundant. Zamia floridana (Fig. 1) locally known as the "coontie," occurs in thickly set clumps as under brush in the open pine woods of south Florida, while Z. pumila is found more sparsely among the denser forest growths of the hammocks.

the cortex supports primarily the primitive outer investiture or armor of old foliar bases, which is quite persistent in some forms, but in others is more or less rapidly excised by the formation of successive layers of periderm, at first arising within the leaf-bases, and then in the cortex itself with the casting off of thin The fern-like and subterranean stemmed Stan geria paradoxa is abundant both in the open grassy veldt and in the bush of Zululand. Simi larly the low-growing Macrosamia spiralis and Bowenia serrulata together form a moderately close undergrowth in the Eucalyptus bush of southeastern Australia. Along the eastern mountain slopes of the Mexican state of Vera Cruz, the tall Dion spinulosum is in places the only large plant and may be said to form a cycad forest. But as a rule, the cycads now play a rather inconspicuous role in forest facies. (See Geographic Distribution). The stem consists finally in a thin zone of wood, cam bium and bast, enclosing a large medulla and enveloped by a thick cortex, this being in the main the arrangement in all the gymnosperms and the dicotyls. But there is this difference; bark. Moreover, in Cycas, Macrosamta and Encephalartos the woody cylinder does not as in the other genera remain single. After a time the primary cambium becomes inactive, and there successively arise in the cortex secondary cambiums of diminishing power and regularity. From these are produced the so-called 'anoma lous wood zones," rarely increasing to a dozen and finding no parallel amongst modern plants. The principal features of this second or polyxy lic trunk type are shown in Fig. 2, together with a medullar system of anastomosing cauline bun dles. These also occur in the pith of Enceph alartos. Both pith and cortex are traversed by anastomosing mucilage canals. The cortical bundle system is a complex one, varying greatly in the several genera, and including "girdle leaf (cf. g, Fig. 5) of a primitive, partly concentric structure. The leaf traces are always double and the two branches may nearly girdle the stem. They are most nearly direct in their course from the woody cylinder through the cortex to the leaf base to Zamia and Stangeria. Complexity of the leaf traces begins even in the cotyledonary plate, but is partly a result of the thickness of cortex.

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