The cycads, unlike all vascular cryptogams, send down a primary root which continues as a tap-root. In the case of such subterranean trunks as those of certain species of Zamia (Fig. 1), the tap-root remains prominent, and its lateral branches are relatively small, the trunk assuming a carrot-like form. But in most genera the root system comes to be quite fila mentous, being largely made up of freely branching adventitious roots. In Cycas the rootlets come to the surface as a felt-like mass and bear numerous coraloid tubercles, recalling the nitrogen fixing tubercles of legumes. The leaves or fronds are usually of two kinds, scale and foliage. There are also the fertile or car pellary leaves of Cycas. All appear in terminal rosettes in the order named, scale, foliage, and when present, carpellary leaves. The scale leaves are dry and aborted foliage leaves, and are present in all but certain species Macro amnia. The elliptical to acunuinate foliage leaves are bipinnate in Bowenia, and pinnate in all the other genera. Prefoliation is direct in Dion and Macrosatnia. In Cycas the swirls is straight, but the pinnules are circinnately rolled like those of ferns. Conversely in several other genera the pinnules lie straight along the once deflected rachis. The pinnules vary from broad forms of papery thinness in some Zamias to leathery and hard spinose types like Encepk alartos. They vary much in size and form and are of dichotomous venation, except in Cycas, which has a single mid-vein. Those of Stangeria are very fern-like, and Cycas Micho litsii is bilobately dichotomous, signally recall ing the fossil Ginkgoid leaves, especially of the Rhitic. The anatomical structure is much as in certain conifers and the extinct Cordaites.
Fructification.— The living cycads are all dicecious. The microsporophylls are always organized into cones which may vary from a few centimeters long to enormous forms ex ceeding in size the very largest ears of Indian corn, and bearing as many as 600 microsporo phylls, these being the most massive to be seen in seed plants. In number the staminate cones vary from a few, or even a single subterminal cone, to over 100 • laterally borne cones in the Australian Macrostionia Moorei. The sporangia are borne on the under side of the sporophyll and are structurally much like those of Angiop teris among the Marattiacece. In certain genera there is an obscure grouping into sori. (See Fig. 1). The ovules, which are the largest known in the vegetable kingdom, are in the genus Cycas, doubtless the most primitive type among recent phanerogams, borne on the margins of modified leaves, emergent in regular series like the ordinary foliage leaves. In the other genera the megasporophylls each bear but a single pair of ovules, and are organized into terminal, sub terminal or lateral cones. These are of striking size in all the genera, and reach a length of nearly a meter with a weight of 90 pounds in some Australian Macrozarnias. In various forms following development of a single cone, the stem is prolonged from a new lateral bud as a sympode. But cones truly lateral occur in both Macrosantia and Bowenia, often in some number. The seeds are often of large size. In Cycas especially• the free seeds look much Bice large plums; but in cone-bearing forms there are strong appression faces. The seed coat is amphivascular— that is, with a mid stone separating an inner and outer bundle .
system. Cycas platyspermic, and the other genera have radio-symmetric seeds. There are from one to three cotyledons. Testal structures
are closely analogous to those of Ginkgo and the Cordaitales. Even stronger is the resem blance to the seeds of the Paleozoic quasi-ferns, or Cycadofilices, if, as seems necessary, the apical testal division seen in these ancient seeds is regarded as primitive. The seeds of cycads, like those of Ginkgo, are edible. They even form for a considerable part of the year the main food of some of the native Australians although it is said that as freshly gathered the seeds contain a poisonous principle. This is removed by maceration and drying.
Fecundation.— Among the various primitive characters of the cycads going to prove their descent from homosporous tree-ferns, easily the most recondite is the occurrence of motile multiciliate male cells of the coiled type, charac teristic of all the Pteridophytes except the club mosses. These spermatozoids were doubtless common to all the cycadales and are present in Ginkgo, but are not known in any other phanerogams. The pollen grains are first drawn through the micropylar tube into the pollen chamber (by suction), after which the pollen tube ruptures the exine and enters the nucellar tissue, where it may branch. Meanwhile the spermatozoids (Figs. 6, 7) form from the gen erative cells and, after the rupture of the pollen tube, swim actively to the archegonium through a liquid medium, afforded in part by the tube and probably also by extrusion from the egg cell. The mature spermatozoids are the largest known in any plant (or animal), at least in Zamia floridana. In this species they are visible to the naked eye, and have been studied alive in sugar solutions. They are of nearly spherical top-shape, with a ciliferous spiral running from the apex to the middle region, and motion is mainly by means of their cilia, but there is also amceboid motion of the spiral end (see Figs. 6 and 7). From this most primitive form of fecundation known in flowering plants it seems evident that not until the later stages of plant evolution did the pollen tube begin to serve as a direct means of transfer of the male cells as in other phanerogams.
Classification and Geographic Distribution. —The living cycads include a few over 100 species divided into two families, the Cycadece, with the single genus Cycas, and the Zamice, including the other genera. The species are nearly equally divided between the New and Old World thermal tropics. There are four indi genous Occidental genera: Zamia, with nearly 40 species, ranges over the mainland from Florida, where two species occur, to Brazil, and also over the West Indies; Ceratozamia, with six species, and Dion, with three, are mainly Mexican and Central American; while the monotypic Microcycas is Cuban. In the Orient five genera occur: Cycas, with over 20 species, is the most notable, and ranges over Australia, the East Indies and Japan; Macrozamia, with 12 species, and the monotypic Bowenia are both strictly Australasian; while Encephalartos, with 20 species, and the monotypic Stangeria are African. This represents a peculiarly balanced distribution. No genus is common to both the Orient and Occident. Mexico, Africa and Aus tralia are the main centres of present day cycads; and while Cycas is the only eastern form extending into the northern, Zamia is the only western form known to extend into the southern hemisphere. There is, however, no reason to believe the species are now under going restriction. The eastern Andine base is the portion of the cycad habitat least known.