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Specialized Vegetative Structures
Introduction: The specialized vegetative structures include runners, suckers, crown, offsets, bulbs, corms, tubers, tuberous roots and stems, rhizomes and pseudo bulbs that function primarily in the storage of food, nutrients and water during adverse environmental conditions. Plants possessing these modified plant parts are generally herbaceous perennials, in which the shoot die down at the end of a growing season but the fleshy vegetative structure usually do not die and remains in the soil, which put forth new vegetative growth in the next season. Plants that survive as underground storage organs are called geotypes, which can withstand period of adverse growing conditions in their regular growth cycle. A brief description on propagation techniques used for species with specialized organs is as under: Bulb: A bulb is a complicated structure, which consists of a short thickened stem bearing roots on the underside and thick fleshy leaves on the upper side. Bulbs are usually produced by monocotyledonous plants in which these are modified for storage of food and water and reproduction. Oxalis, is the one dicot genus that produces bulbs. Bulbs generally consist of bulb scales Fig.14.1 and Fig.14.2. The outer bulb scales are fleshy, which contain reserve food materials, where as inner scales do not function as storage organs and are leaf like. In the centre of the bulb, there is either a vegetative meristem or an unexpanded flowering shoot. Meristem develops in the axil of these scales to produce miniature bulbs, known as bulblets, which when grown to full size are known as offsets. In some species (e.g. Lilies), the bulblets are produced in the underground organs. Bulbs are of two type’s tunicate i.e. (laminate) and non-tunicate (scaly).Fig. 14.1: Bulb of onion Fig. 14.2: Scaly bulb of Lilium halandium Non-tunicate (scaly) bulbs: Non-tunicate bulbs are represented by the lily, have dry coverings, loosely arranged like the petals of a flower overlapping each other. In general, non- tunicate bulbs are easily damaged and must be handled more carefully. They must be kept moist because they are injured by drying. The bulbs are propagated through offsets, bulb cuttings, scaling, scoring, stem cuttings, leaf cuttings, leaf buds and bulbils. In case of scaling, individual bulb scales are separated from the mother bulb and placed 5-6 cm deep vertically in a suitable growing media. These scales usually develop 3-5 bulblets after 4-5 weeks of planting. In many advanced countries, there is a programme to produce forcible commercial bulbs after one growing season only from scaling with preformed bulblets in lilies. Basically four types of plant development take place in this method of scaling as under:In scoring, three straight cut are given across the basal plate and growing point of the bulb but the complete plate is not removed as in scooping. Growing point in the axils of bulb scales grow into bulb lets. After scoring and scooping, the bulbs are placed about 2.5 cm deep in dry sand or open trays at about 21oC for callusing. After callusing, the bulbs are incubated in trays at 21oC, which may be increased 29-32oC in dark or diffused light for 2 weeks and held at 80-90 percent relative humidity for 2-3 months. The mother bulb along with bulblets are planted 10cm deep in the nursery in the fall. Fig.14.3: Stem tuber of potato Runners: It is a specified long slender stem that develops from the axil of leaf at the crown of a plant, which prostate horizontally along the ground. The roots appear at one of the nodes having contact with soil. After root formation in the new plant, the contact with the mother plant is automatically detached and the new plant can be separated and planted. Strawberry is a typical runner( Fig.14.4). Oxalis corniculata, Bosten fern, bugle (Ajuga) and spider plant (Chlorophytum comosum) are other examples of plants propagated through runners. Fig.14.4. Production of strawberry runners Fig.14.5. Sward suckers production in banana Fig. 14.6: Off-set production in pineapple Fig. 14.7: A rhizome showing the emergence of aerial shoots Corms: A corm is a short, solid, much swollen underground stem, enclosed by dry scale like leaves, with one or several buds near the top, a tuft of leaves at the upper side and a ring of thick fibrous roots around the base (Fig.14.8). At the apex of corm is terminal shoot, which usually develops into a flowering shoot or leaves. Gladiolus, crocus and water chestnut are examples of some typical cormous plants. Cormous plants can be propagated through new corms, cormels or corm segments. Usually, a mother corm produces 2-3 new corms and 15-20 cormlets, the miniature corms. Fig. 14.8: Corm of Saffron Pseudo bulbs: Pseudobulb (false bulb) is an enlarged fleshy stem with several nodesFig.14.9). It is produced by some horticultural plants (mostly orchids), mainly for the storage of food. The plants producing pseudobulbs are usually propagated through offshoots, division or from black and green bulbs produced by them. The rooted shoots are cut from the mother bulb and planted in the nursery. Some orchids, like Cattleya, Laclia, Miltonia etc. are multiplied commercially through the division of pseudobulbs. The bulb is cut into different sections by a sharp knife during dormant season. Each section is sown and new growth begins at the nodes. Fig.14.9: Pseudo bulbs |
Last modified: Wednesday, 19 September 2012, 9:08 AM