NEET Biology Notes Plant Kingdom Gymnosperms
- These are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds that develop post-fertilisation are not covered, i.e. are naked.
- These include medium-sized trees or tall trees and shrubs. Roots are generally tap roots. In some genera, roots have fungal association in the form of mycorrhiza (Pinus). In some others (Cycas) small specialised roots called coralloid roots are associated with’ N2 -fixing cyanobacteria. In gymnosperms, fertilisation occurs by siphonogamy eliminating the dependence upon external water. The stems are branched (Cycas) or unbranched (Pinus and Cedrus).
- The leaves may be simple or compound. In conifers, the needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.
- The cones or strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili. The microspores develop into a male gametophytic generation, which is highly reduced called as pollen grain. The cones bearing megasporophylls with ovules or megasporangia are called macrosporangia or female strobili.
- The male or female cones may be borne on the same tree [Pinus). In Cycas, these are borne on different trees. The ovules are borne on megasporophylls, which may be clustered to form the female cones.
- The megaspore mother cell divides meiotically to form four megaspores. One of the megaspores enclosed within the megasporangium (nucellus) develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs.
- The pollen grain is released from the microsporangium. The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia. Zygote develops into an embryo and the ovules into seeds. These seeds are not covered
- It has 20 species over world out of which 6 are reported from India. Cycas revoluta or sagopalm is most commonly cultivated. It’s roots are of two types, i.e. tap roots and coralloid roots. The primary root is tap root, which gives rise to many lateral branches and are responsible for absorption of water and minerals. The coralloid roots, can be apogeotropic, ageotropic or negatively geotropic. They have no root hair and root caps. Blue-green algae like Nostoc, Anabaena are present in algal zone of these roots and fixes atmospheric nitrogen.
- Stem is thick, unbranched with a crown of leaves at the apex and covered by persistent and woody triangular leaf bases all over its surface. Stele in stem is eustele with conjoint, collateral and open vascular bundles arranged in a ring. Xylem is made up of tracheids only, phloem is composed of sieve tubes and phloem parenchyma only.
- The wood is monoxylic (non-porous) and polyxylic. Leaves are dimorphic, one scale and other foliage. Foliage leaves are unipinnately compound, show circinate vernation and presence of ramenta when young. Rachis has omega-like formation of diploxylic vascular bundles. Vascular bundles in leaflet are also diploxylic (centripetal as well as centrifugal).
- It is present in leaflet between spongy and palisade parenchyma and acts in lateral conduction of water. Vegetative reproduction takes place by bulbils formed on stem. Cycas is heterothallic and dioecious plant. The male cone develops at the apex of stem in between crown of foliage leaves. It consists of several microsporophylls arranged spirally around the cone axis. Microsporophyll is differentiated into proximal wedge-shaped fertile part and distal sterile part tapering into an upcurved apophysis. The groups of microsporangia called sori are present on the abaxial side of fertile part of microsporophyll.
- The development of microsporangium is eusporangiate type. Each microsporangium is an oval body with short stalk, single layered jacket, several wall layers and single layered tapetum (which provide nourishment).
- Microspore mother cells inside microsporangium undergo meiosis to form tetrahedrally arranged haploid microspores. The microspore develops exine and intine and divide to form prothalial cell, generative cell and tube cell. The megasporophylls are spirally and loosely arranged at the tip of female plant. Each megasporophyll has upper broad sterile and lower fertile structure, which bears 2-10 ovules laterally.
- Ovules of Cycas are large, naked, orthotropous, unitegumic (i.e. single integument differentiated into outer fleshy sarcotesta, middle stony sclerotesta and inner fleshy sarcotesta).
- It serves as megasporangium. One of its cell undergoes meiosis and form four megaspores, three degenerate and one becomes functional. The functional megaspore forms female gametophyte. .
- The nucellus protrudes into the micropyle forming nucellar beak having pollen chamber (pollens are deposited here after pollination). The ripe orange coloured ovules secrete a nectar drop, which help in catching of pollen grains. Pollination is anemophilous and direct. The exine of pollen rupture, intine elongate to form pollen tube, which act as haustorium and absorbs nutrients.
- The generative cell divide to form a stalk cell and body cell. The body cell divides and form top-shaped, multiciliate (spiral band of cilia) antherozoids. The megaspore undergoes repeated divisions and forms multicellular female gametophyte, which serves as endosperm (haploid). The archegonial initials (peripheral cells) develop into archegonia. Each archegonium has short two celled neck, a large egg and a venter canal cell (neck canal ceils are absent).
- Fertilisation is siphonogamous. The diploid zygote develops into embryo. The embryo consists of radicle, plumule, haustorium, suspensor and two cotyledons.
- Mature plant of Pinus is large tree and is differentiated into thick columnar stem, evergreen leaves and roots. The main trunk represents the stem, which is cylindrical and branched. The branches are confined to upper part of the stem, while the apical bud of main stem grows faster then the apical buds of branches. The branches are arranged in acropetal order thus giving the pyramid or conical shaped appearance to the tree. The branches are of two types, i.e. long shoot that are unlimited in growths and dwarf shoot (spur) that develop in the axile of scaly leaves. The leaves are of two types, i.e. scale leaf and foliage leaf. .
- The male cone develops below the apical bud on long shoot in cluster. Each male cone has 60-125 microsporophylls arranged spirally. Each microsporophyll has microsporangium that bears microspores (pollen grains). Female cone is present in axile of scale leaf. On the central axis are borne two kinds of paired scales in spiral fashion
(megasporophylls). Two naked ovules are borne on the upper surface of ovuliferous scales (upper scale of megasporophyll), towards the end, which is near the central axis.
- Distal sterile, broad part of ovuliferous scale is called apophysis. Pollination takes place by wind. About 15 months are required between pollination and fertilisation. Pinus plants are sources of different commercial products like, timbers, resins, chilgoza, etc.