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STEM Bud The stem, together with leaves and flowers, develops from terminal bud which is an immature shoot. The stem is more complex than root: Presence of nodes and internodes; Terminal bud produces leaves, flowers and axillary buds giving branches (root apex does not produce lateral organs). The stem functions as support and conducting; sometimes also as storage and photosynthetic organ. Leaves are involved only in photosynthesis. Bud •Terminal bud produces primary meristems: protoderm, procambium and fundamental meristem, which in turn produce adult tissues (primary thickening) such as epidermis, vascular bundles and fundamental tissues, respectively. •Terminal bud is a dynamic structure producing continuously young leaves and lateral buds which in turn have the same pattern of development described above. Primary thickening of stem •It is due to the intense cell division activity of apical bud producing new cells. As the latter move away from the apex they undergo elongation and differentation giving primary growth of stem. Stem Types of primary thickening Gymnosperms Vascular system forms a continuous circle around the pith Angiosperm Dicotyledones Vascular bundles form a discontinuous ring around the pith Angiosperm Monocotyledones Vascular bundles scattered in the whole parenchyma •Primary thickening of linden •Primary thickening of Dicotyledones and Monocotyledones •Primary thickening of Dicotyledones Vascular bundles •Vascular bundles bearing procambium residues are called opened and give rise to secondary thickening (secondary xylem and phloem). Conversely, they are called closed and did not give raise to secondary structure (e.g. Monocotyledones and herbaceous Dicotyledones). • Each bundle is surrounded by a sheath, composed of sclerenchyma fibres, that controls movement of substances and protects conducting cells. Closed vascular bundle t.s. of stem of Monocotyledone Stem modifications •Tuber is a swollen stem that functions as an underground storage organ. Stem tubers, for example potatoes, often produce buds along the stem from which aerial stems arise the following season. Root tubers produce no buds. Potato tubers Stem modifications •Bulb is an underground storage organ, comprising a short, flattened stem with roots on its lower surface, and above it fleshy leaves or leaf bases, surrounded by protective scale leaves, and one or more buds often enclosed in protective scales. It may provide the means for vegetative reproduction, or for the survival of the plant from one season to the next. Garlic bulb Stem modifications Turmeric rhizome •Rhizome is a horizontally creeping, underground, root-like stem which bears buds or shoots and adventitious roots and usually persists from season to season. Stem modifications •Stolones are stems growing horizontally at ground level, rooting at the nodes and producing new plants from its buds (e.g. vegetative reproduction). Stem modifications Cladode or phylloclades of asparagus, branches taking on the form and functions of a leaf Cladodes of Epiphyllum Stem modifications •Thorns of Crataegus are modified branches, woody, projecting structures with a sharp point that are derived from the stem, or branch of the plant and is connected to its vascular system. Pharmaceutical importance of stem •Stem provides numerous plant drugs through bark (e.g. cinnamom, Rhamnaceae, cinchona tree), rhizome (e.g. rhubarb, ginger etc.), resin (e.g. conifers), latex (e.g. Ficus, rubber) and exudates (e.g. arabic gum, myrrh).