Gymnosperms Sentence Examples
Gymnosperms were the highest types of plants.
The older types of Gymnosperms are inelastic and dying out.
There is no doubt that the phylum of Angiosperms has not sprung from that of Gymnosperms.
Our knowledge of the Gnetales leaves much to be desired, but such facts as we possess would seem to indicate that this group is of special importance as foreshadowing, more than any other Gymnosperms, the Angiospermous type.
The genus Ephedra, with its prothallus and archegonia, which are similar to those of other Gymnosperms, may be safely regarded as the most primitive of the Gnetales.Advertisement
It is not improbable that the three genera of this ancient phylum survive as types of a blindly-ending branch of the Gymnosperms; but be that as it may, it is in the Gnetales more than in any other Gymnosperms that we find features which help us to obtain a dim prospect of the lines along which the Angiosperms may have been evolved.
The megaspore is filled with tissue as in typical Gymnosperms, and from some of the superficial cells 3 to 5 archegonia are developed, characterized by long multicellular necks.
They thus differ considerably from the cones of other members of the order Coniferae, of Gymnosperms, to which the junipers belong.
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.
The ovary enlarges, and, with the seeds enclosed, constitutes the fruit, frequently incorporated with which are other parts of the flower, as receptacle, calyx, &c. In gymnosperms the pollen-tubes, having penetrated a certain distance down the tissue of the nucellus, are usually arrested in growth for a longer or shorter period, sometimes nearly a year.Advertisement
At a more recent horizon, the silicified specimens of the Mesozoic Gymnosperms from Great Britain, France, and especially North America, are no less important.
On the other hand, fern-like seed-plants, known as Pteridosperms, and Gymnosperms belonging almost entirely to families now extinct, were abundant, while the Pteridophyta attained a development exceeding anything that they can now show.
We must bear in mind that throughout the Palaeozoic period, and indeed far beyond it, vascular plants, so far as the existing evidence shows, were represented only by the Pteridophyta, Pteridosperms and Gymnosperms. Although the history of the Angiosperms may probably go much further back than present records show, there is no reason to suppose that they were present, as such, amongst the Palaeozoic vegetation.
Consequently, the Pteridophytes, Gymnosperms and their allies had the field to themselves, so far as regards the higher plants, and filled places in nature which have now for the most part been seized on by families of more modern origin.
The roots were at first like those of Marattiaceae but grew in thickness like the roots of Gymnosperms.Advertisement
The presence of Cordaitean leaves indicates that Gymnosperms of high organization already existed, a striking fact, showing the immense antiquity of this class compared with the angiospermous flowering plants.
Of Gymnosperms we have Cordaitean leaves, and the stems known as Pitys, which probably belonged to the same family.
Cordaites, Dorycordaites and many stems of the Mesoxylon type represent Gymnosperms; the seeds of Pteridosperms and Cordaiteae begin to be common.
Starting with the Permo-Carboniferous vegetation, and omitting for the moment the Glossopteris flora, we find a comparatively homogeneous flora of wide geographical range, consisting to a large extent of arborescent lycopods, calamites, and other vascular cryptogams, plants which occupied a place comparable with that of Gymnosperms and Angiosperms in our modern forests; with these were other types of the greatest phylogenetic importance, which serve as finger-posts pointing to lines of evolution of which we have but the faintest signs among existing plants.
Other types, again, which may be referred to the Gymnosperms, played a not unimportant part in the Palaeozoic vegetation.Advertisement
A flora consisting entirely, with a single doubtful exception, of Gymnosperms and Cryptogams gives place to one containing many flowering plants; and these increase so rapidly that before long they seem to have crowded out many of the earlier types, and to have themselves become the dominant forms. Not only do Angiosperms suddenly become dominant in all known plantbearing deposits of Upper Cretaceous age, but strangely enough the earliest found seem to belong to living orders, and commonly have been referred to existing genera.
The plant-bearing deposits next in age, which have yielded Angiosperms, appear to belong to the Cenomanian, though from Westphalia a few species belonging to the Cryptogams and Gymnosperms, found in deposits correlated with the Gault, have been described by Hosius and von der Marck.
The absence of Gymnosperms is noticeable.
The British Eocene and Oligocene strata yield so large a flora, and contain plant-beds belonging to so many different stages, that it is unfortunate we have still no monograph on the subject, the one commenced by Ettingshausen and Gardner in 1879 having reached no farther than gocene 79 g Oli of Great the Ferns and Gymnosperms. This deficiency makes it impossible to deal adequately with the British Eocene plants, most of the material being either unpublished or needing re-examination.
Among the 200 plants of the London Clay are no Ferns, but 6 genera of Gymnosperms - viz.Advertisement
As showing the richness of this flora, we may mention that in the only orders which have yet been monographed, Ferns are represented by 17 species and Gymnosperms by 10, though these are not the groups best represented.
The plants include a Fern, Onoclea hebridica, close to a living American form; four Gymnosperms belonging to the genera Cryptomeria, Ginkgo, Taxus and Podocarpus; Dicotyledons of about 30 species, several of which have been figured.
The Irish strata yield two ferns; 7 Gymnosperms, Cupressus, Cryptomeria, Taxus, Podocarpus, Pinus (2 species), Tsuga; and leaves of about 25 Dicotyledons.
Among its Gymnosperms are numerous Cupressineae of African affinity belonging to the genera Callitris and Widdringtonia, and a juniper close to one indigenous in Greece.
To date three volumes have been published covering ferns and fern allies, orchids and gymnosperms and non-orchid monocotyledon.
Gymnosperms have mainly paternal (pollen) transmission while most flowering plants seem to have maternal inheritance.
During recent years a number of fossil (Carboniferous and Permian) plants have been very thoroughly investigated in the light of modern anatomical knowledge, and as a result it has become st i s clear that in those times a large series of plants etisted ear ys intermediate in structure between the modern ferns tern of Cycaand the modern Gymnosperms (especially Cycads), dofiices.
Between these two extremes is every conceivable gradation, embracing aquatic and terrestrial herbs, creeping, erect or climbing in habit, shrubs and trees, and representing a much greater variety than is to be found in the other subdivision of seed-plants, the Gymnosperms.
As in Gymnosperms, branching is monopodial; dichotomy or the forking of the growing point into two equivalent branches which replace the main stem, is absent both in the case able variety in form (see Leaf), but are generally small in comparison with the size of the plant; exceptions occur in some Monocotyledons, e.g.
Before leaving the pollen-sac a division takes place in the pollengrain into a vegetative cell or cells, from which the tube is developed, and a generative cell, which ultimately divides to form the male cells (see Angiosperms and Gymnosperms).
The Lyginodendreae type of structure, however, appears to have formed the transition not only to the Cycadales, but also to the extinct family Cordaiteae, the characteristic Palaeozoic Gymnosperms (see p. 107).
The history of the Ginkgoales will be found in the Mesozoic section of this article (see also Gymnosperms); their nearest Palaeozoic representatives " were probably members of the Cordaitales, an extinct stock with which the Ginkgoaceae are closely connected " (Seward).
From Jurassic rocks of France and Italy a few imperfect specimens have been described as carpels of Cycads, like those of the recent genus Cycas (see Gymnosperms); while a few of these may have been correctly identified, an inspection of some of the original examples in the Paris collections leads one to express the opinion that others are too imperfect to determine.
The living Maidenhair-tree (Ginkgo biloba) (see Gymnosperms) remains, like Matonia and Dipteris, among the ferns, as an isolated relic in the midst of recent vegetation.
In Gymnosperms, where vessels and fibres are absent, the late summer wood is composed of radially narrow thick-walled tracheids, the wood of the succeeding spring being wide-celled and thin-walled, so that the limit of the years growth is very well marked.
The formation of additional cambial cylinders or bands occurs in the most various families of Dicotyledons and in some Gymnosperms. They may arise in the pericycle or endocycle of the stele, in the cortex of the stem, or in the parenchyma of the secondary xylem or phloem.
Filicales and Gymnosperms, and known as the Cycadofihices, a group in which, curiously enough, the reproductive organs remained undiscovered for some time after the anatomy of the vegetative organs was sufficiently well known to afford clear evidence of their true affinities.
In many Gymnosperms the male nucleus penetrates the female nucleus before fusing with it (Blackman, Ikeno).
Companion cells are not found in the Pteridophyta and Gymnosperms. In the latter their place is taken by certain cells of the medullary rays and bast parenchyma.
Both were in turn replaced by the Lower Mesozoic flora, which again is thought to have had its birth in the hypothetical Gondwana land, and in which Gymnosperms played the leading part formerly taken by vascular Cryptogams. The abundance of Cycadean plants is one of its most striking features.
As in other parts of the earth, the Triassic was the age of gymnosperms, which were represented by diverse types.
As Coulter and Chamberlain express it, " the habitats of the Gymnosperms to-day indicate that they either are not at home in the more genial conditions affected by Angiosperms, or have not been able to maintain themselves in competition with this group of plants."
The best known and by far the largest division of the Gymnosperms is that of the cone-bearing trees (pines, firs, cedars, larches, &c.), which play a prominent part in the vegetation of the present day, especially in the higher latitudes of the northern hemisphere; certain members of this class are of considerable antiquity, but the conifers as a whole are still vigorous and show but little sign of decadence.
Brongniart, who was the first to investigate in detail the anatomy of a cycadean stem, recognized an agreement, as regards the secondary wood, with Dicotyledons and Gymnosperms, rather than with MonocoFIG.8.
These forms will, however, be found discussed in the articles treating of extinct plants and the Gymnosperms, but their recognition will serve to emphasize, in conclusion, the important position the Pteridophyta hold with regard to the existing flora.
For an account of the further development of the megaspore, and the formation of the egg-cell, from which after fertilization is formed the embryo, see Gymnosperms and Angiosperms.
A great group of Palaeozoic fossils, showing evident affinity to Ferns, has proved to consist of seed-bearing plants allied to Gymnosperms, especially Cycads.
There appears, in fact, so far as stem-structure is concerned, to have been no sharp break between the typical Palaeozoic Gymnosperms and pronounced Pteridosperms such as Lyginodendron.
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.
The union of the germ nuclei has now been observed in all the main groups of Angiosperms, Gymnosperms, Ferns, Mosses, Algae and Fungi, and presents a striking resemblance in all.
Moreover there is the fact that the flowers of nearly all the primitive phanerogams, such as the Gymnosperms, consist solely of sporophylls, having no perianth.
It was characterized by arborescent vascular Cryptogams and Gymnosperms of a type (Cordaiteae) which have left no descendants beyond it.
Its use with any approach to its modern scope only became possible after Robert Brown had established in 1827 the existence of truly naked seeds in the Cycadeae and Coniferae, entitling them to be correctly called Gymnosperms. From that time onwards, so long as these Gymnosperms were, as was usual, reckoned as dicotyledonous flowering plants, the term Angiosperm was used antithetically by botanical writers, but with varying limitation, as a group-name for other dicotyledonous plants.
The advent in 1351 of Hofmeister's brilliant discovery of the changes proceeding in the embryo-sac of flowering plants, and his determination of the correct relationships of these with the Cryptogamia, fixed the true position of Gymnosperms as a class distinct from Dicotyledons, and the term Angiosperm then gradually came to be accepted as the suitable designation for the whole of the flowering plants other than Gymnosperms, and as including therefore the classes of Dicotyledons and Monocotyledons.
These, as in Gymnosperms, are of two kinds, microspores or pollen-grains, borne in the stamens (or microsporophylls) and megaspores, in which the egg-cell is developed, contained in the ovule, which is borne enclosed in the carpel (or megasporophyll).
The development of the ovule, which represents the embryo- Gymnosperms; when mature it consists of one or two sac. coats surrounding the central nucellus, except at the apex where an opening, the micropyle, is left.
As in Gymnosperms and other groups an interesting qualitative change is associated with the process of fertilization.
In Gymnosperms we have seeds, and the carpels may become modified and close around these, as in Pinus, during the process of ripening to form an imitation of a box-like fruit which subsequently opening allows the seeds to escape; but there is never in them the closed ovary investing from the outset the ovules, and ultimately forming the ground-work of the fruit.
Fungi Algae Bryophyta Pteridophyta Phanerogamia Gymnosperms Angiosperms Algae in this wide sense may be briefly described as the aggregate of those simpler forms of plant life usually devoid, like the rest of the Thallophyta, of differentiation into root, stem and leaf; but, unlike other Thallophyta, possessed of a colouring matter;.
Among Gymnosperms the secondary xylem is similarly simple, consisting of tracheids which act as stereom as well as hydrom, and a little amylom; while the phloem-parenchyma sometimes undergoes a differentiation, part being developed as amylom, part as proteid cells immediately associated with the sieve-tube, in other cases the proteid cells of the secondary phloem do not form part of the phloem-parenchyma, but occupy the top and bottom cellrows of the medullary rays, the middle rows consisting of ordinary starchy cells.