Biology XI:3 Plant Kingdom(4 Division Gymnospermae)

Division Gymnospermae Word gymnosperms, gymnos − naked, sperma − seeds Ovules not enclosed by any ovary wall Seeds formed after fertilization are not covered (i.e., naked). Include medium-sized trees, shrubs, and tall trees Contains the world’s largest plant Sequoia - the giant redwood Plants have tap roots. Roots in some genera show symbiotic associations. Mycorrhiza shows association of fungi with Pinus roots. Coralloid roots of Cycas show association withN2-fixing Cyanobacteria. Leaves are well-adapted to withstand extreme conditions. In conifers, needle-like leaves with thick cuticle and sunken stomata reduce surface area and water loss. Spores produced within sporangia, borne on sporophylls, which form strobili or cones Male and female strobili may be borne on same tree (Pinus) or on different trees (Cycas). Megaspore mother cell divides mieotically to form four megaspores. Megaspore mother cell is a differentiated cell of nucellus. Nucellus protected by envelopes is known as an ovule. Male and female gametophytes do not have independent existence, hence remain within sporangia. Steps in fertilization: Pollen grain released from microsporangium ↓ Carried by air currents ↓ Come in contact with ovules ↓ Discharge of pollen content on mouth of archegonia ↓ Fertilization ↓ Formation of zygote ↓ Development of naked seed For Example − Pinus and Cycas