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Term Paper on Gymnosperms
Term Paper Contents:
- Term Paper on the General Consideration of Gymnosperms
- Term Paper on the Origin of Gymnosperms
- Term Paper on the Characteristics of the Principal Orders of Gymnosperms
- Term Paper on the Classification of Gymnosperms
- Term Paper on the Economic Importance of Gymnosperms
Term Paper # 1.
General Consideration of Gymnosperms:
The idea of gymnospermy, as opposed to angiospermy, was first proposed by Robert Brown (1827). Since then, the question which is not still clearly answered is whether the group as a whole is monophyletic or polyphyletic in origin.
The majority of workers, however, are of opinion that it is not monophyletic. Further as the gymnosperms have been distinguished from the angiosperms mainly on the basis of the seed-character alone, they are to be considered as constituting an artificial group.
Whatever may be the validity or nature of the origin of the group, the gymnosperms or ‘naked-seeded’ plants are the most primitive of all seed-plants, as indisputable records of gymnosperms have been found in the Devonian (perhaps in the Silurian as well), whereas the angiospermous remains have been recorded only from the Jurassic onwards. They are of importance in the modern flora as they are well represented in the Himalayas and the coniferous vegetation is of importance as it produces timber and resin.
The gymnosperms are regarded as an intermediate group between the pteridophytes and angiosperms. The plant always represents the sporophyte, and is heterosporous. The sporophylls are generally aggregated into cone-like structures or strobili which are usually unisexual, and do not bear any accessory whorl.
In the gymnosperms, the carpels do not unite to form a closed chamber or ovary, but the mega-sporangium or the ovule is borne directly on the surface of the megsporophyll as in the case of pteridophytes, and the pollination is direct, i.e., the pollen grains are transferred directly on the micropyle of the ovule.
As in the heterosporous pteridophytes, two kinds of gametophytes are produced but they are highly reduced as in angiosperms. Archegonia are still present. The fertilization is effected by means of motile spermatozoids in lower gymnosperms, but in higher ones by means of non-motile gametes.
Unlike the angiosperms, the endosperm is normally formed directly from the megaspore before fertilization is effected; as such it is a haploid tissue and is regarded as the female gametophyte in gymnosperms. As the carpel or carpels do not unite to form an ovary, the seeds are naked, and there is no such organ as fruit in the gymnosperms.
Term Paper # 2.
Origin of Gymnosperms:
When the anatomical and morphological features of the gymnosperms are taken into account, the division breaks itself into two separate groups, viz., and cycadophytic and coniferophytic lines. The cycadophytic line contains the Pteridosperms, Cycads and Cycadeoids, and it shows evidences of derivation from the ferns.
The coniferophytic one contains the Cordaites, Ginkgos and the Conifers. Its origin is unknown. The oldest known Coniferophyta belong to the Pityeae (upper Devonian). Pityeae were succeeded by the Cordaiteae (Upper Carboniferous) from which the Ginkgoales and Coniferales were probably derived during the late Paleozoic (Permian).
The Cycadophyta show evidence of fern ancestry. The oldest Pteridosperms appear in lower Carboniferous and oldest Cycads in late Paleozoic. Origin of Cycadeoideales, Ephedrales and Gnetales is not clear.
Term Paper # 3.
Characteristics of the Principal Orders of Gymnosperms:
1. Pteridospermae (Cycadofilicales):
Trees or smaller plants with fern-like foliage. Sporangia are borne on frond-like leaves more or less modified, but never grouped into a cone.
2. Bennettitales:
Trees with stems covered with an armour of persistent leaf-bases. Microsporophylls frond-like forming a loose crown. Megasporophylls no longer leaf-like but found in a specialized cone-like structure.
3. Cycadales:
Plants with palm-like habit. Micro-and megasporoylls grouped into cones, except in the genus Cycas, where the megasporophylls are collected into a loose crown as in the Bennettitales. Male cells motile.
4. Pentoxylales:
Low shrubs. Female infructescence consisting of numerous cones specially attached on peduncles, each bearing numerous ovules on a central axis, with no megasporophylls, ovuliferous scales or interseminal scales. Male flowers terminally borne on dwarf shoots. Unilocular sporangia terminate short-branchlets of the appendages or microsporophylls.
5. Cordaitales:
Large trees with flat strap-shaped leaves. Fructifications in cones.
6. Coniferales:
Large trees or shrubs. Leaves needle or scale- like, sometimes flattened. The male flowers are arranged in more or less compact clusters or catkins and the female flowers are usually clustered to form a cone. Male cells never motile.
7. Ginkgoales:
Medium-sized trees. Leaves flattened and lobed in various ways, with dichotomous venation. Male and female elements collected into strobili.
8. Gnetales:
Small trees or shrubs or climbing shrubs. Opposite leaves, dicotyledonous embryos and compound cones in both male and female are the characteristics of the order.
Plants are relatively small, usually unbranched or poorly branched, aerial or subterranean trunk. Their leaves are usually large and pinnate. The stems have a large pith, scanty wood, usually with wide medullary rays and a thick cortex.
There are three orders under the division, viz.,
a. Cycadofilicales,
b. Bennettitales and
c. Cycadales.
The first two are extinct and the last one is still living.
Term Paper # 4. Classification of Gymnosperms:
Of the different systems of classification, the one proposed by Prof. Chamberlain is adopted here. He divides the group into Cycadophyta and Coniferophyta, basing on the differences in the morphological and anatomical characters.
The former section is characterized by plants of comparatively smaller size with un-branched stems, pinnately compound leaves, thick cortex, scanty wood, large pith, and generally sporophylls are in simple cones.
The Coniferophyta, on the other hand, are marked by having comparatively larger plants with extensively branched stems, simple leaves, scanty cortex, thick wood, small pith and usually the microsporophylls form simple cones and the megasporophylls form compound ones. Compound leaf is an outstanding character of Cycadophyta and the simple leaf of Coniferophyta.
The Cycadophytes are divided into three orders:
(1) Cycadofilicales (Pteridospermae) (Carboniferous, extinct),
(2) Bennettitales (Mesozoic, extinct), and
(3) Cycadales (Mesozoic to present day).
On the other hand, the Coniferophytes include four orders:
(1) Cordaitales (Paleozoic, extinct),
(2) Ginkgoales (Mesozoic to present day),
(3) Coniferales (Permian to present day), and
(4) Gnetales (recent).
Prof. Sahni (1948) reported a unique group of Gymnosperms the Pentoxyleae from the Rajmahal Hills, Bihar. The group has a combination of characters suggestive of the affinities with the Cycadales, the Bennettitales and the Coniferales.
A new classification of Gymnosperms has been suggested by Pant (1957).
A brief synopsis of Pant’s scheme is given below:
Division 1. Cycadophyte:
Plants with relatively small, usually unbranched or poorly branched, aerial or subterranean trunk, their leaves are usually large and pinnate. The stems have a large pith, scanty wood, usually with wide medullary rays and a thick cortex.
Class 1. Pteridospermopsida:
Order 1. Lyginopteridales
Order 2. Medullosales
Order 3. Glossopteridales
Order 4. Peltaspermales
Order 5. Corystospermales
Order 6. Caytoniales
Class 2. Cycadopsida:
Order 7. Cycadales
Class 3. Pentoxylopsida:
Order 8. Pentoxylales
Class 4. Cycadeoideopsida (Bennettitopsida):
Order 9. Cycadeoideales (Bennettiales)
Division 2. Chlamydospermaphyta:
Peculiar angiosperm-like forms.
Class 1. Gnetopsida:
Order 1. Gnetales
Order 2. Welwitschiales
Division 3. Coniferophyta:
Usually large-sized trees with profusely branched stem and with simple leaves. Their stems have a small pith, abundant wood, usually with narrow medullary rays and a narrow cortex.
Class 1. Coniferopsida:
Order 1. Cordaitales
Order 2. Coniferales
Order 3. Ginkgoales
Class 2. Ephedropsida:
Order 4. Ephedrales
Class 3. Taxopsida:
Order 5. Taxales
Term Paper # 5. Economic Importance of Gymnosperms:
The members of the gymnosperms are utilized for various purposes. The chief uses of them are, however, as fuel, timber, lumber and materials for making building and furniture. Some of them are used for ceremonial decorations as well as ornamental garden plants. The tubers of the cycads yield the arrowroot starch.
Some of the pines are sources of the pulp for the paper industry. Previously, the pine wood was employed in the preparation of match sticks. Some of the valuable pine products, which are obtained by distillation and other processes, are methyl alcohol, charcoal, resin, wood gas, turpentine, various kinds of oil and others.
The bark of hemlock (Tsuga canadensis) gives tannins, which are extensively used for the purpose of tanning animal skins into leather, for making ink, as well as for manufacturing drugs. Pinus gerardiana and several others supply edible seeds.
Amber, which is utilized for making ornaments and art curios, is nothing but the resin of a fossil pine. An important medicinal plant is Ephedra, from which is obtained the alkaloid ‘ephedrine’. The inflorescence and young leaves of G. gnemon are taken as vegetables in some parts of South-East Asia.
The seeds also are edible after cooking or roasting. From the bark is obtained a kind of fibre, which is used for making ropes, fishing nets, etc. In South India the seeds of G. ula are taken as food. The plant also possesses some properties detrimental to the fishes.
The kernel of the seed of G. latifolia is also taken as a food after boiling or roasting. Its bark is also utilized for preparation of nets and ropes. The fossil gymnosperms, particularly the Pteridosperms, have contributed to a large extent to the formation of coal.
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