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| Aerial Stem Modifications |
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| In some plants the aerial stem is modified to perform a variety of special functions. The aerial stem modifications are as follows: |
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| Tendrils develop as modifications of the stem in certain plants. The terminal bud gives rise to a tendril in Cissus quandrangularis and the axillary bud becomes modified into a tendirl in Passiflora. |
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| fig. 27.26 Auxillary Stem Tendrils of Passiflora |
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| The thorn is a hard, straight, and pointed structure. In Bougainvillea and Duranta, the axillary bud is modified into a thorn. In Carissa the terminal bud is modified into a pair of thorns. The thorn sometimes bears leaves, flowers and fruits as seen in Duranta and Pomegranate. The thorns not only check the rate of transpiration but also protect the plants from herbivore grazing. Eg. Carissa, Duranta, Citrus |
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| fig. 27.27 Stem Thorns |
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| Thorns are small, modified stems, spines are modified superficial leaves and prickles are sharp, pointed structures without vascular tissue, arising on stem, petioles and inflorescence. |
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| A phylloclade is a flattened stem of several internodes functioning as a leaf. In Opuntia the stem is modified into a green flattened structure called Phylloclade. On the surface of the phylloclade, clusters of spines are formed. These spines are the modified leaves of the axillary bud. These spines not only check the rate of transpiration but also protect the plant from herbivores. The phylloclade has distinct nodes and internodes. E.g., Opuntia |
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| In Muehlenbeckia the stem is a thin, green, flat structure made up of nodes and internodes. Initially small normal leaves are formed but later only scale leaves. |
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| fig. 27.28 Phylloclade |
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| A phylloclade of one or two internode is called as a cladode. There are no suitable examples of cladodes because, Ruscus and Asparagus, which are often considered as cladodes, are in reality, cladophylls. A cladophyll is a flattened leaf like stem arising in the axils of a minute, bract-like, true leaf. |
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| fig. 27.29 - Cladodes |
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| Bulbil is a speical multicellular body essentially meant for reproduction. In Agave the floral buds are modified into bulbils. These bulbils get detached, come in contact with the soil and develop into new plants. In Dioscorea the axillary bud develops into a bulbil. This bulbil detaches from the mother plant and grows up into a new independent one. |
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| fig. 27.30 Bulbil |
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| The various modifications seen in the stem are summarized in the following Table. |
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| fig. 27.31 |
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