GYMNOSPERM NEB BIOLOGY
Gymnosperm are the naked seeded plant i.e. seeds are not enclosed by definite covering. The plant body is sporophyte and is differentiated into roots, stem and leaves. The reproductive organs are borne in strobili or cones (except the megasporophyll of Cycas). Gymnosperms are mostly evergreen but few deciduous plants are also known. The leaves may be either large fern like (cycadophytes) or small or simple (coniferophytes) eg. Cycas, Pinus
The Pinus is evergreen, perennial and xerophytic plant. The Pinus tree is sporophyte and differentiated into root, stem and leaves. It is main source of resins and wood. In Nepal, there are two naturally growing species of Pinus i.e. Pinus roxburghii and pinus wallichiana. Leaves are modified into pine needles and also possesses sunken stomata in the epidermal layer of the needle are the main xerophytic characters of it.
It possesses a prominent tap root which doesn’t penetrate deep in the soil. It develops the lateral roots which forms the massive root system. When the root system is well developed it forms the mycorrizhal root i.e. root shows the symbiotic association with fungus. It provides some growth hormone to the plant and also helps in water absorption.
The main stem is erect, cylindrical, and woody and branched. The branches are of two types- Long shoot (branch of unlimited growth) and dwarf shoot (branch of limited growth). The long branches are present on the main stem and possesses apical bud hence are of unlimited growth. Short branches has an ephemeral terminal bud, grows for short period. These branches arise from the long shoot and possess green needles. Plants are dimorphic (having two types of leaves) possesses scale leaves and foliage leaves. Scale leaves are thin, small, dark brown borne on both long and dwarf shoot. These fall off with the maturity of the branches. Foliage leaves are borne only in dwarf shoot. They are long, narrow, needle like, green and frequently called as pine needles. The dwarf shoot bearing needles are known as spurs. The needles are persistent. It is xerophytic adaptation of the plants.
Reproduction in Pinus
Pinus is a monoecious plant i.e. male and female cones are found in the same plant. The plant reproduces by asexual spore which are produced in sporangia. The spores are of two types: microspore and megaspores borne on microsporophyll and megasporophyll respectively.
Male cone of Pinus
Male cones arise in clusters in place of dwarf shoots. Each cone consists of numerous microsporophylls, arranged spirally on the central axis. Two microsporangia are present on the lower side of each microsporophyll. Inside the sporangia a large number of microspore mother cells are present which undergo reductional division and produced haploid microspores (pollen grains).
Female cone of Pinus
Female cones are borne at the tips of long shoots and generally occur in pairs. Each cone consist of a central axis to which are attached the megasporophylls. Each megasporophyll on its upper surface has two ovules with small bract scale and a large ovuliferous scale. Each ovule consists of nucellus surrounded by a single integument. Within the nucellus a megaspore mother cell undergoes two successive divisions (reductional) and forms a linear tetrad of megaspores. Out of four only one megaspore is functional. The remaining three simply degenerates soon. This megaspore develops into the female gametophyte. It contains two archegonia which bear egg. After fertilization the ovules developed into seed which are naked and endospermic. It has many cotyledons. The seed on germination produces the sporophytic plant.
Cycas grows naturally in tropical and subtropical places of the eastern hemispheres both in the hills and plains. It prefers open and sloppy areas where the habitat in various parts of world. The genus Cycas is primitive gymnosperms. The general appearance of plant is like a small palm tree. Cycas pectinata is found in Nepal. Cycas revolute is also cultivated at places. Their growth is very slow and can live for several hundred years.
Cycas is an evergreen sporophyte. It bears a crown of large foliage leaves on the top of a trunk. It is differentiated into stem, leaves and roots.
The cycas plant usually consists of an unbranched trunk. The whole stem is covered by persistent leaf bases. The diameter of trunk may be up to 2m. Cycas possesses two types of leaves. A rosette of scale leaves is always present around the growing point of stem. The bases of scales leaves are persistent. The foliage leaves from a dominant crown at the top of short trunk. The foliage leaves are very large. They are petiolated and pinnately compound. The petiole bears two rows of spines.
Large no. of leaflets arranged in two lateral rows on either side. Each leaflet has a mid-rib but no veins. The young leaves show circinate venation. Cycas possesses root hair and root caps. The coralloid root devoid of root hair and root caps. They show dichotomous branching and negatively geotrophic. They possess the BGA (Anabaena cicadae, Nostoc punctiformae) helps in nitrogen fixation.
Reproduction of Cycas
Cycas is a dioecious plant and heterosporous. The pollen sacs and ovules are borne on scale like microsporophylls and megasporophylls respectively. The sporophylls grow in group and may form cone or strobilus. A strobilus is a group of sporophylls tightly packed around a central axis.
Male cone (microsporangiate strobilus)
The male cone develops singly at the tip of stem, in the centre of crown of leaves. It is shortly stalked, compact, oval or conical structure. The cone consists of a large number of microsporophyll or stamens spirally arranged on the central axis of cone. Microsporangia or pollen sacs are found on the lower surface of microsporophyll. They are grouped in sori (sing. Sorus). Each sorus consists of two to six mircrosporangia. Microspores or pollen grains are produced in microsporangium after reductional division. On maturity of the microsporangia, the microspores released from longitudinal slit, pollen grains germinate developing pollen tube. The pollen tube develops to male gametophyte with spermatozoids.
Megasporophyll of Cycas
It does not bear female cone. The megasporophylls are arranged spirally and arranged loosely like the foliage leaves. The megasporophyll may contain flat upper part with many leaflets and the lower parts with several marginal ovules, 1to 6 pairs, arranged alternately. The ovules and sporophylls are covered by hairs. The ovules are orthotropus consist of female gametophyte (prothallus) embedded in nucellus and covered by the integuments. One of the cells of nucellus functions as megaspore mother cell which undergoes reductional division and gives rise to four megaspores. Out of the four megaspores, three degenerate and one remains functional which develop the female gametophyte. The gametophyte contains two to eight archegonia towards micropyle.
Economic importance of gymnosperms
- Many gymnosperms (Cycas rvoluta, huja, Araucaria etc.) are usually cultivated in the gardens as ornamentals.
- Many conifers (Pine, Cedar, Hemlock etc.) are great importance for their light colored, straight grained and light weighed and softwood.
- Seeds of many cycads and conifers are edible.
- Species of Ephedra yield an alkaloid called ephedrine and used in the preparation of medicines for the treatment of cough, asthma and bronchitis.
- Many gymnosperms (Agathis Alba, Pinus) are the source of resins and turpentine.
- Many gumnosperms yield essential oils which are used in perfumery and as flavouring agent.