Organization of Cycadeoidea.— In the Mesozoic forests in which they grow, the ma ture trunks of Cycadeoidea, with their foliage, would have appeared to a beholder to differ little from modern cycads. Some of the low branched Dakotan trunks must have greatly resembled the Uganda Encephalartos of Fig. 4. Only when in full fruit would essential differ ences have appeared to the eye. Vegetatively, the more copious fern-like chaff or ramentum borne by the leaf bases forms the chief outer peculiarity. As in the cycad trunk, the diver gence from other gymnosperms is mainly due to the retention of the old leaf bases, the heavy underlying cortical, parenchyma and the pres ence of a relatively enormous medulla or pith. The woody cylinder is thus relatively reduced; it is also of somewhat simple structure and usually without growth rings. There is, how ever, a tendency to the formation of growth rings in some of the silicified trunks, with de velopment of a woody cylinder as heavy as in Cordaites and a correspondent reduction of both the medulla and cortical region; it being noteworthy that these variant features are par alleled by the existing Dion. In its simplest form the woody cylinder of the Cycadeoids (and cycads) consists in a regularly aligned lat tice of anastomosing collateral bundles. The meshes are formed by the leaf gaps. Each cylinder bundle is divided into a conspicuous xylem and phloem region separated by a well marked cambium. The xylem or secondary wood tracheids are interspersed by occasional manner as in the fossil forms, where the petiolar bundles are more simply aligned. In cycads the petioles have a prominent median ridge with pinnule insertion in two lateral fur rows; but most fossil forms appear to have parenchyma cells. The °medullary rays° are abundant, deep and made up of sheets of oblong to squarish cells, often two cells thick. Next the pith, the spiral protoxylem first arises as more or less isolated strands, soon merging into the main body of scalariform secondary wood. While the scalariform pits of the mid-region of the tracheids tend to break up into a radial circular pitting of the Araucarioxlylon type, it may be said that in nearly all the species the main body of wood is characteristically scalar iform. Only in one species is the circular•pit ting found to form a thin outer zone next to the cambium. Essentially this same variation is found in the wood of modern cycads, except that in the latter there is more development of pitted wood. The cycadeoids thus appear to have been a primitive scalariform type, while the cycads seem to advance toward a pitted condition. But this difference is partly the result of post-Cretaceous change. In both groups the phloem region agrees in a regular alternation of thin and thick walled elements. From the cylinder meshes, or leaf gaps, arise the leaf traces which are single and pass out directly to the leaf bases.
The leaves of the cycadeoids are in venation reminiscent of the Cordaites. As in that group and the cycads the bundles are mesarch. The inverted omega bundle pattern of the latter, as seen in the transverse petiolar sections, is some what unique, but varies in much the same had a more or less pronounced median furrow, on the sides of which the pinnules were obliquely inserted. Prefoliation of the fossil types was direct, with pinnule growth some what preceding rachidal elongation as in the existing Macrozamia spirali. See Fig. 9.
The flowers of a number of species of Cyca deoidea are known in various stages of growth from initial or minute forms a few millimeters in diameter to the mature ovulate cones or fruits. All the strobili, whatever the stage of growth, indicate more or less clearly an am phisporangiate condition; but there is conclu sive evidence for both bisexual and monoxious species,. with a probability that certain others di were cecious. Briefly, the flower is a short lateral axillary shoot, always invested in a husk of hairy bracts (cf. figures 11-13). At the centre is an immature ovulate cone, envel oped by a staminate disc. The buds vary from a few centimeters long by a half centimeter in diameter, to forms 5 to 10 centimeters in di ameter. (Fig. 11). The marvelously perfect silicification of the flowers is due to the pro tected position in which they grew packed in amongst the old leaf bases; but even so, the fossilization of such delicate organs as sta mens and pollen must be regarded as fortui tous. In species where the peduncles elongated no fruits were left behind. The great number of fruits borne by various mature trunks is a striking feature, indicating in several of the species monocarpy, or fruit growth for a sin gle season only, followed by sterility and death. The stamens are as in the far more reduced amphisporangiate flower of Tumboa, united into a campanulate disc, hypogynously in serted. The border of the campanula rises to
nearly the height of the immature central cone and then splits into discrete sporophylls. Each of these then sends up two wing-like spurs and at the same time the rachidal tips turn inward and downward along the surface of the cone— that is, the component fronds of the flower small grain of rye to a large grain of wheat, while the infertile scales rise about and top this seed, so that their expanded ends come bud are of deflexed or conduplicate preflora tion. The sporophyll spurs rise closely ap pressed each to each, forming instead of a rounded summit a spired dome inside the bract husk. The outer surface is toward the apex quite tomentose. Each stamen is once pinnate with the pinnules reduced and bearing two lateral rows of pendent bivalvate synangia, varying from a few to 10 or 12 to the row. As densely packed in the flower-bud, the syn angia show strong appression faces (Figs. 12, 13), but are of essentially the same structure as in the existing tree ferns of the genus Marattia. The sporangia in two facing rows of 10 or 12 each resting in the synangial valves are often filled choke-full of well-conserved pollen. The grains are much like those of ex isting cycads, though tending to reach a slight ly larger size. It is noteworthy that some of the discs are considerably reduced and that re lated forms of the Mexican Liassic have a nearly simple campanula. See series of Figs. 11-14.
The mature ovulate cone (Fig. 10) is of in verted pear shape and consists in a flattened to conical receptacle, bearing a closely interlocked series of sterile and fertile scales. At the pe riphery all the scales are sterile, but in all the central parts of the cone, as so clearly seen in the transverse sections, the fertile scales are so distributed that each is surrounded by some regular number, as from five to six, or five to seven of the more flattened infertile scales. Each megasporophyll bears a single erect and terminal seed, generally from the size of a to surround the micropylar tubes in a series of interlocking rosettes. The seeds are radiosper mic, five to six angled like those of Palxozoic rather than Mesozoic times, exalbuminous or nearly so, and dicotyledonous. The testa is rather reduced, triple-layered with a middle stone one or two cells thick and non-vascular. These are the only known fossil dicotyledonous seeds, but the embryos are conserved in beautiful detail. In form and testal structure, including peculiarities of the micropylar tube, the Cyca deoidea seed presents certain striking approxi demarcation, stands the related and far greater group of dissociated fossil remains conveniently termed the Williamsonian Tribe. This tribe, taking the term in its very broadest sense, thus designates that dominant and varied but im perfectly known assemblage of Mesozoic plants, to which the silicified cycadeoids afford the structural key. These are, too, the proangio sperms, as they were with much foresight called by the French palwobotanist .Saporta. Obviously, accessions of petrified forms are few; but the discovery of related casts and im prints is only begun. The latter are in a sense the more important, since they in the end af ford the chief evidence for the variety and ex tent of extinct cycadophytan vegetation. The first attempt at the reconstitution of a William sonian 'cycadeoid was that of Zamites, or, as now called, Williamsonia gigas, by the Eng lish palzeobotanist, W. C. Williamson (1870). But his conclusions, based on the casts and imprints of stems, leaves and fruits• from the Yorkshire coast, failed to convince contempo rary botanists. With Cycadeoidea understood, it is found, however, that the Williamson res toration only lacked completeness. By far the best-known member •of the Williamsonian tribe is the Wielandiella from the Rhatic of Sweden, as skilfully studied and restored by A. G. Nathorst, and shown in the plate. This plant had a much branched, scandent or de cumbent habit and bore very small perfect flowers with reduced discs. Because of great age and plastic features Wielandiella must be regarded as the most important cycadeoid; but unfortunately the stem structure remains un known. The Pterophyllum Fayoli of the French Carboniferous is one of the oldest and handsomest of the leaf forms; while the In dian (and cosmopolitan) Diayozamites had the significant feature of net-venation. The Williamsonians leave behind approximately the record of emergence and extinction indicated in the appended table. In order to keep the form simple (and more nearly correct), vari ous types doubtless related, such for instance as the Carboniferous to Permian Plagiozain ites, are omitted.