It involves the laying down of extra vascular and strengthening tissue necessitatedīy the increased size of the plant. Marked in those with perennial (persistent) aerial parts (i.e. Secondary thickening occurs to some degree in most dicotyledons and is especially Produce vascular tissue beyond the elongating region until only a narrow The vascular tissues lengthen very rapidly to form the conducting strands.Īt about the same time the first strengthening tissues (sclerenchyma fibers)ĭevelop on the outside of the phloem. In the elongating region the first conducting tissues ( protoxylem and protophloem) are formed from the procambial strands. While certain columns of cells retain their ability to divide and are known The outer cells develop into epidermal cells and very quickly tiny leavesĪrise and cover the growing point. The cells produced by the meristemĭevelop vacuoles and lengthen rapidly, producing an elongation of the stem.Īs the cells lengthen they begin to differentiate, or change in character. Rapidly dividing meristematic cells which are without vacuoles (Fig 3). The growing point in a stem is situated right at the tip. In the axils of the leaves axillary buds develop into new growing The growing point is surrounded and protected by the developing leaves. Growth takes place just behind the tip not along the whole length Soil and branch to form a network of roots. The downward-growing tip will penetrate the Part which grows upwards there is an apical shoot, whose growth increases The embryo that is formed is elongate in shape. When a seed planted in the soil germinates, The bundles are frequently surrounded by sclerenchyma fibers. Strengthening tissue may occur, especially near The bundlesĪre not arranged in any regular pattern and are of the closed type, Monocotyledon stems differ considerably from the foregoing. Rays are normally of simple parenchyma cells. Vessels have numerous unthickened pits through which the Ringed in the first-formed xylem ( protoxylem). Xylem tissues are dead and have lignified These are elongated vessels and shorter tracheids,Īlthough the latter are not common. Part of the bundle consists of the water-conducting tissue called xylem. These play an important part in secondary growth. Cambium is the name given to the activelyĭividing cells or meristems in the center The dicotyledon bundles are said to be open (i.e., they contain cambium). Each sieve tube has a companion cell alongside it which regulates its functioning. The ends of the tubes are perforated ( sieve-plates), allowing It consists of elongated sieve-tubes with large vacuoles. Through which manufactured food is transported. They are tough and elastic, enabling the plant to standīending by the wind. The fibers have no cytoplasm and are therefore Various chemicals known collectively as lignin. These are elongated cells whose walls have been impregnated with The conducting (vascular) tissue is arranged in vascular bundles.Įach bundle has on the outside a number of sclerenchyma fibers. The cortex is the starch sheath whose cells have starch deposits within them. In the outer cortical region the cells have thicker walls for extra supportĪnd are known as collenchyma cells. The cortex cells are simple and of the type known as parenchyma. The epidermal cells are regular and have a waxy cuticle on the outside. Short distance from the tip shows the mature primary structure. A suitable, easily obtained dicotyledon stem for study is that of the sunflower. The internal structure is studied by cutting very thin sections and examining It is in the arrangement of the tissues that roots and Plant and the description given here will cover roots and leaves as well. These are basically the same in all parts of the There are conductingĪnd supporting tissues. The internal structure of the stem reflects the function (Fig 1). It ends in a flower or dies away and a lateral bud assumes the main Well illustrated in larches and "Christmas trees." The cymode branching pattern differs in that the main stem apex does not continue through The main axis of a plant may continue to grow throughout life, giving offīranches laterally. The age of a woody twig can be established When these scale-leaves fall off in spring The outer leaves are modified as protective The leaves are clustered togetherĪround the tip (the growing point). Shoots (stems) which have not yet elongated. These axillary buds often remain dormant but may produceĪ branch, especially if the main stem is damaged. Has one or more leaves, each of which has a bud in its basal angle ( axil). There are several nodes onĪ stem, each separated by leafless internodes. They support the leaves and flowers and they carry water and food from place to place within the plant.Ī typical stem is cylindrical and may be soft ( herbaceous)
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