Gymnosperms Study

Occurrence

Gymnosperms are naked seeded plants. Gynmosperms are a group of ancient plants. They become dominant in the Jurassic period Most of the gymnosperms are evergreen trees. Some shrubby pl..flas are also found in this group. They have worldwide distribution. They are most abundant in the temperate region. The fossils of gymnosperms are found near coal and oil deposits.

Life cycle

Gymnosperm shows a dear alternation of sporophytic and gametophytic generations. The plant body is sporophytic generation. The gametophytic is very much reduced. It had completely lost independent existence.

(a) Sporophyte

The plant body is a sporophyte. It is well differentiated into root, stem and leaves.

1. Roots: Primary or tap root is well developed. Root develops root caps.
2. Leaves: Both simple and compound leaves are found. Scale leaves are associated with the foliage leaves. The foliage leaves are

without a reticulate venation. The branching of the veins is dichotomous type. Their leaves are modified to withstand the xerophytic conditions. The leaves have tough cuticle, sunken stomata and mesarch bundle. Stem: The stem is woody. Vascular bundles are collateral (arrange in rings) and open. They are arranged in a ring in the primary stem. Secondary growth takes plan by the activity of the cambium. It produces the secondary xylem (wood). Xylem is formed of xylem parenchyma and tracheid. They have no true vessels except the order Gnetales The phloem is formed of phloem parenchyma and sieve tubes. But they are without any companion cells. Primary cambium may remain active throughout the life of the plant. Then secondary and tertiary cambia are formed. It produces alternate zones of wood. The activity of phellogen produces bark on the surface.

Cones or Stroblli

Gymnosperms are heterosporous. They produce micro and megaspores. They develop definite unisexual cones or strobili. These cones show a great variation.

(a)  Microspore and microsporangia: The microspores (pollen grains) are produced in microsporangia (anthers). They are borne on microsporophyll (stamens). Microsporophylls are spirally arranged on the central axis of the male cone. Microsporangia have microspore mother cells. Each cells divide by meiosis to form four haploid microspores. Microspores are dispersed by wind. Microspore represents the beginning of the gametophytic stage. It germinates to give rise to the male prothallus within the spore wall. It has only one prothalial cells. It cuts off generative cell. This generative cell represents the reduced antheridium. Generative cell divides into a basal stalk cell and a body cell. Body cell gives rise to two male gametes. Pollen tube carries these male gametes (pollen grain) to the oosphere. Male gametes are non-motile except certain primitive forms as Cycads.

(b)    Megaspores and megasporangia: The megaspores (embryo sac) are produced in megasporangia (ovules). They are borne on the megasporophylls (carpels). The ovules are covered by a single integument. It has single megaspore mother cell. This cell divides meiotically to form four megaspores. These megaspores degenerate only one remains functional. The functional megaspore is the beginning of the gametophytic stage. It germinates to form multicellular female prothallus within the spore wall. Superficial cells gives rives to two or more simple archegonia towards the micropylar end. Each archegonium has a simple neck cells and a venter. The venter contains an egg (oosphere) and a ventral canal cell.

Fertilization and seed formation

Pollen tube carries male gametes to egg (oosphere). Fertilization occurs and diploid oospore is formed. Oospore is the beginning of gametophyte generation. The oospore gives rise to the embryo. Prothalial tissues provide nourishment to developing embryo. Integument is transformed into the seed coat. The unutilized prothalial cell becomes endosperm.

Affinities of Gymnosperms

Gymnosperms are closely related to Pteridophytes and Angiosperms. It forms an intermediate group between the two. Thus gymnosperms have closes affinities with both these groups.

(a) Affinities with Pteridophytes

Resemblances

The primitive gymnosperms like Cycas are much identical with Pteridophytes (ferns). They were taken as Pteridophytes for long time. They were called Ferns with seeds. There are following resemblances between both groups.

1. Both have regular alternation sporophytic and the gametophytic generations. Their sporophyte is dominant and forms the main plant body. Gametophyte is reduced to prothellus.
2. Both are heterosporous.
3. Both have similarities in external morphology. Their bodies are differentiated into root, stem and leaves.
4. Both have vascular elements. i.e., xylem and phloem. Vessels and companion cells are absent in both.
5. Both show circinate ptyxis (Cycads).
6. The sporangia of both are arranged in son, as in Cycas.
7. The megaspore is retained within the megasporangium.
8. Development of the embryo and the formation of a suspensor as in Selaginella.

Differences

1.1n Pteridophyta, both the microspores and megaspores are shed from the sporangia. The gametophytes develop as independent individual plants in water or on soil. In gymnosperms, only the

1.microspores are shed from the sporophyte. But the megaspore remains in the megasporangia.

2. There is no seed formation in the Pteridophytes. But present in gymnosperms.
3. The male cells or sperms are carried by pollen-tube to the archegonia in the gymnosperms. But pollen tube is absent in Pteridophytes.
4. The gametophytes of gymnosperms are simple and not green in colour as in fern or Equisetum.
5. There are no neck canal cells and sometimes also no venter canal cell, and the neck is very small in gymnosperms.
6. Secondary growth occurs in gymnosperms. But it is absent in Pteridophytes due to absence of cambium in gymnosperms.
7. There is a tap-root system as compared to the adventitious root system of Pteridophytes.

(b) Affinities with Angiosperms

Resemblances

1. They are similar in their external morphology, i.e., the differentiation into root stem and leaves.
2. Both have identical internal anatomy. Cambium is present in gymnosperm and dicot angiosperms.
3. The cones of gymnosperm and flower of angiosperms have close evolutionary link.
4. Both develop pollen tube for the transfer of male gametes.
5. Both have permanent retention of the megaspore or embryo sac cell in the megasporangium (ovule).
6. Both produce ovules and seeds.
7. Both have similarities in alternation of generation. Sporophyte is dominant in both cases. Differences
8. The reproductive structures of angiosperms are flowers, those of gymnosperms are cones.
9. In angiosperms, the seeds are enclosed by true carpels and at maturity, a carpel forms a fruit. It is absent in gymnosperms.
10. Angiosperms show double fertilization. But only single fertilization occurs in gymnosperms.
11. The endosperm in angiosperms is triploid. The endosperm in gymnosperms is haploid.
12. In gymnosperms pollen-grains land directly upon the exposed

ovules. In angiosperms, the ovules are enclosed by the carpels. Therefore, pollen-grains land upon the apex of the pistil called stigma.

6. In gymnosperms, the pollen-grain during germination forms prothalial cell or cells. But these are not produced in angiosperms.
7. Angiosperms lack archegonia and antheridia
8. The xylem in angiosperms has vessels and phloem have companion cells. But these are absent in gymnosperms.

Classification of Gymnosperms

Bentham and Hooker divided the Gymnosperms into three groups: Cycadaceae, Gnetaceae and conifers. Coulter divided gymnosperm into six orders:

1. Cycadofilicales
2. Bennettitales
3. Cycadales
4. Cordaitales
5. G inokoa les
6. Gnetales