What are non flowering plants?
The most successful group of land plants possesses seeds. The seed is a fertilized ovule. The ovule, which develops into seed, has many advantages related with it. The female gametophyte is protected by the ovule and is totally dependent upon the parent sporophyte. After fertilization it develops a food storage tissue, called female prothallus or endosperm. The seed probably arose independently at different times in the evolution of plants. Some fossil 'ferns' have been found with structures that resemble seeds of the spermatophytes (seed bearing plants). Present-day seed plants are classified into two groups-gymnosperms and angiosperms. Seed plants without flower which we are going to discuss hear on the head of non flowering plants.
GYMNOSPERMS : the non flowering plants
Gymnosperms are the most ancient group of seed plants, originated in the Palaeozoic era. The geological records show that they were leading plants over the earth's surface during the Jurassic and Cretaceous periods of the Mesozoic era. Gymnosperms are characterized by the presence of naked ovules, borne unguarded on the surface of the megasporophylls. Hence, unlike angiosperms, seeds of gymnosperms are not enclosed in ovary. Gymnosperms are sometimes called 'Phanerogams with no ovary'. There are about 70 genera and more than 700 species of living non flowering plants, distributed throughout the temperate and tropical regions of the world. In general, gymnosperms are woody trees or shrubs. The main plant body is sporophytic and is differentiated into root, stem and leaves. The plant possesses a well developed tap root system. The roots are same as polyarch. The stem is erect and branched . The xylem consists of only tracheids and xylem parenchyma; vessels are though absent with the exception of Gnetales. The phloem consists of sieve tubes and phloem parenchyma. Most gymnospermic leaves do not have lateral veins and the lateral teans-location of nutrients takes place with the help of transfusion tissue present in the leaves. In most non flowering plants reproductive organs are aggregated into compact cones. The cones are generally monosporangiate. Male cones are usually short-lived. Microsporangia develop on the abaxial side of the microsporophylls.
Female cones are long lived and are aggregation of megasporophylls which bear megasporangium i.e. ovule. The ovule is orthotropous and unitegmic with the exception of Gnetales. The ovular integument is differentiated into an outer and an inner fleshy layer and a middle stony layer. On the other hand, the male gametes are non-motile but in Cycas and Ginkgo they are flagellated. The megaspore in the ovule develops into female gametophyte (embryo-sac) within the nucleus. An archegonium has a single egg and a venter canal cell; neck canal cells are, however, absent. Pollination is direct, i.e., pollen grains come in direct contact with the ovule. Pollens are deposited in the pollen chamber where they germinate. All gymnosperms are wind-pollinated. Fertilization is siphonogamic and the pollen tube may function as sperm carrier. Development of endosperm takes place before fertilization and hence the endosperm is haploid. There is a noticeable tendency for polyembryony and several embryos develop in a female gametophyte. The uncovered ovule develops into seed. The seed usually has a short to long resting period and then it germinates to form a new sporophytic plant.
Economic Importance of non flowering plants
Apart from ornamental value, for which the non flowering plants are planted in gardens, they are of great economic value as they provide many useful products of commercial importance. Conifers provide enormous amount of soft wood for construction, packing, plywood, board and paper industries. Turpentine, resin, tannins and several other products are also obtained from conifers. Fossilised resin of Pinus succinifera, known as amber, is of great commercial value. The seeds of Pinus gerardiana are roasted and may be eaten. The leaves of Taxus baccata are used in treatment of asthma, bronchitis, epilepsy and for indigestions. This non flowering plant is also the source of an anticancer drug. Another non flowering plant specis, Ephedra are the source of a valuable drug, ephedrine which is used in the treatment of cold, hay fever, asthma and other respiratory diseases.