Word Finder - cycads

definition

Any plant of the division Cycadophyta, having a stout and woody trunk with a crown of large, hard and stiff, evergreen leaves.

The stem resembled that of Cycads in having a large pith, sometimes as much as 4 in.

We now know that many at least of the Cycadofilices bore seeds, of a type much more complex than that of most modern seed plants, and in some cases approximating to the seeds of existing Cycads.

In Monoblepliaris, one of the lower Fungi, in some Algae, in the Vascular Cryptograms, in Cycads (Zamia and Cycas), and in Ginkgo, an isolated genus of Gymnosperms, the male cell is a motile spermatozoid with two or more cilia.

This body has been called a blepharoplast, and in the Pteridophytes, Cycads and Ginkgo it gives rise to the spiral band on which the cilia are formed.

Amongst palms the Corypheae are represented by Sabal and Thrinax, and there is a solitary Za,nza amongst Cycads.

Cycads are represented by Cycas itself, which in several species ranges from southern India to Polynesia.

Amongst Cycads, Zamia is confined to the New World, and amongst Conifers, Araucaria, limited to the southern hemisphere, has scarcely less antiquity; Pinus reaches as far south as Cuba and Nicaragua.

Conifers are scantily represented by Callitris and Podocarpus, which is common to all three sub-regions; and Cycads by the endemic Encephalartos and Stangeria.

Of Cycads, Australia and New Caledonia have Cycas, and the former the endemic Macrozamia and Bowenia.

A distinct connexion between the flora of the peninsula and Ceylon and that of eastern tropical Africa is observable not only in the great similarity of many of the more truly tropical forms, and the identity of families and genera found in both regions, but in a more remarkable manner in the likeness of the mountain flora of this part of Africa to that of the peninsula, in which several species occur believed to be identical with Abyssinian forms. This connexion is further established by the absence from both areas of oaks, conifers and cycads, which, as regards the first two families, is a remarkable feature of the flora of the peninsula and Ceylon, as the mountains rise to elevations in which both of them are abundant to the north and east.

They are characterized by fine cycads (Zamites arcticus and Glossozamites Hoheneggeri), which also occur in the Urgonian strata of Wernsdorff.

Cordaites, a tall plant (20-30 ft.) with yucca-like leaves, was related to the cycads and conifers; the catkin-like inflorescence, which bore yew-like berries, is called Cardiocarpus.

Among other trees and shrubs may be mentioned the sumach, the date-palm, the plantain, various bamboos, cycads and the dwarf-palm, the last of which grows in some parts of Sicily more profusely than anywhere else, and in the desolate region in the south-west yields almost the only vegetable product of importance.

Certain species are regularly found in the intercellular spaces of higher plants; such are species of Nostoc in the thallus of Anthoceros, the leaves of Azolla and the roots of Cycads.

The plants of the Lower Gondwanas consist chiefly of acrogens(Equisetaceae and ferns)and gymnogens (cycads and conifers), the former being the more abundant.

The same classes of plants occur in the Upper Gondwanas; but there the proportions are reversed, the conifers, and still more the cycads, being more numerous than the ferns, whilst the Equisetaceae are but sparingly found.

It consists partly of sandstones with impressions of plants (cycads, ferns, &c.), and partly of clay-beds with coal.

It was the English botanist Robert Brown who first recognized this important distinguishing feature in conifers and cycads in 1825; he established the gymnospermy of these seed-bearing classes as distinct from the angiospermy of the monocotyledons and dicotyledons.

The line of descent of recent cycads is comparatively clear in so far as they have undoubted affinity with Palaeozoic plants which combined cycadean and filicinean features; but opinion is much more divided as to the nature of the phylum from which the conifers are derived.

The cycads constitute a homogeneous group of a few living members confined to tropical and sub-tropical regions.

In the genus Cycas the female flower is peculiar among cycads in consisting of a terminal crown of separate leaf-like carpels several inches in length; the apical portion of each carpellary leaf may be broadly triangular in form, and deeply dissected on the margins into narrow woolly appendages like rudimentary pinnae.

The male flower of Cycas conforms to the type of structure characteristic of the cycads, and consists of a long cone of numerous sporophylls bearing many oval pollen-sacs on their lower faces.

The stems of cycads are often described as unbranched; it is true that in comparison with conifers, in which the numerous branches, Stem springing from the main stem, give a characteristic form to the tree, the tuberous or columnar stem of the Cyca daceae constitutes a striking distinguishing feature.

The Cyas type of frond, except as regards the presence of a midrib in each pinna, characterizes the cycads generally, except Bowenia and Stangeria.

A feature of interest in connexion with the phylogeny of cycads is the presence of long hairs clothing the scale-leaves, and forming a cap on the summit of the stem-apex or attached to the bases of petioles; on some fossil cycadean plants these outgrowths have the form of scales, and are identical in structure with the ramenta (paleae) of the majority of ferns.

The male flowers of cycads are constructed on a uniform plan, and in all cases consist of an axis bearing crowded, spirally disposed sporophylls.

Webber, and more recent work enables us to assume that all cycads produce ciliated male gametes.

Pollination in cycads has always been described as anemophilous, but according to recent observations by Pearson on South African species it seems probable that, at least in some cases, the pollen is conveyed to the ovules by animal agency.

The anatomical structure of the vegetative organs of recent cycads is of special interest as affording important evidence of relationship with extinct types, and with other groups of recent plants.

The leaftraces of cycads are remarkable both on account of their course and their anatomy.

The complicated girdle-like course is characteristic of the leaf-traces of most recent cycads, but in some cases, e.g.

Similarly in the sporophylls of some cycads the bundles are endarch near the base and mesarch near the distal end of the stamen or carpel.

This is of interest from the point of view of the comparison of recent cycads with extinct species (Bennettites), in which the leaf-traces follow a much more direct course than in modern cycads.

The mesarch structure of the leaf-bundles is met with in a less pronounced form in the flower peduncles of some cycads.

Palaeozoic genera, has not entirely disappeared from the stems of modern cycads; but the mesarch bundle is now confined to the leaves and peduncles.

The roots of some cycads resemble the stems in producing several cambiumrings; they possess 2 to 8 protoxylem-groups, and are characterized by a broad pericyclic zone.

A common phenomenon in cycads is the production of roots which grow upwards (apogeotropic), and appear as coralline branched structures above the level of the ground; some of the cortical cells of these roots are hypertrophied, and contain numerous filaments of blue-green Algae (Nostocaceae), which live as endoparasites in the cell-cavities.

Surrounding the pitted wall of the ovum there is a definite layer of large cells, no doubt representing a tapetum, which, as in cycads and conifers, plays an important part in nourishing the growing egg-cell.

A point of anatomical interest is the occurrence in the vascular bundles of the cotyledons, scale-leaves, and elsewhere of a few centripetally developed tracheids, which give to the xylem-strands a mesarch structure such as characterizes the foliar bundles of cycads.

The leaves at once invite a comparison with ferns; the numerous long hairs which form a delicate woolly covering on young leaves recall the hairs of certain ferns, but agree more closely with the long filamentous hairs of recent cycads.

The spermatozoids constitute the most striking link with both cycads and ferns.

The structure of the seed, the presence of two neck-cells in the archegonia, the late development of the embryo, the partially-fused cotyledons and certain anatomical characters, are features common to Ginkgo and the cycads.

Robert Brown was the first to give a clear description of the morphology of the Abietineous cone in which carpels bear naked ovules; he recognized gymnospermy as an important distinguishing feature in conifers as well as in cycads.

Cordaites is an extinct type which in certain respects resembles Ginkgo, cycads and the Araucarieae, but its agreement with true conifers is probably too remote to justify our attri buting much weight to the bearing of the morphology of its female flowers on the interpretation of that of the Coniferae.

During the growth of the cell which forms the megaspore the greater part of the nucellus is absorbed, except the apical portion, which persists as a cone above the megaspore; the partial disorganization of some of the cells in the centre of the nucellar cone forms an irregular cavity, which may be compared with the larger pollen-chamber of Ginkgo and the cycads.

The most important positive evidence on this point indicates that the most ancient Gymnosperms were derived from the Filicales rather than from any other phylum of the Vascular Cryptogams. Extinct forms are known intermediate between the Ferns and the Cycads, and a number of these have been shown to bear seeds and must be classed as Pteridospermae.

A great group of Palaeozoic fossils, showing evident affinity to Ferns, has proved to consist of seed-bearing plants allied to Gymnosperms, especially Cycads.

We will begin with the Lyginodendreae, a group in which the anatomical characters indicated a systematic position between Ferns and Cycads, long before the reproductive organs were discovered.

The leaf-traces, where they traverse the cortex, have the structure of the foliar bundles in Cycads, for they are of the collateral type, and their xylem is mesarch, the spiral elements lying in the interior of the ligneous strand.

Heterangium shows, on the whole, a decided preponderance of Filicinean vegetative characters, though in the leaf-traces and the secondary tissues the Cycads are approached.