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Monday, 3 December 2018

Growth in Plants

Plants grow from seed to seedlings and in turn grow into mature plants. A germinating seed grows by the lengthening of its radicle and plumule. The root and stem apices of a plant can be divided into the region of cell division, followed by the region of cell elongation and the region of cell maturation. The region of cell division is also known as the apical meristem. It consist of meristematic cells, I.e. cells capable of active division. In this region of cell elongation, the cells become enlarged to their maximum size by the stretching of their walls. This is done by the absorption of water and the incorporation of more cellulose material in the cell walls. The cells in the region of maturation have attained their permanent size and become specialized to Carry out certain functions.

Note: Stem apices include terminal buds and the lateral and axillary buds.

     The apical Meristem are divided into three regions, with reach region giving rise to particular parts of the plant.

     The outermost region in the root gives rise to the piliferous layer and the root Cap. Some of the cells in the piliferous layer extend to from root hairs. In the stem, the outermost region forms the epidermis.

    The middle region gives rise to the cortex and the endodermis. The cortex is a packing tissue. Excess food is usually stored in cortical cells, especially in the root.

     The innermost region gives rise to the pericycle, cambuim, vascular tissue and pith. The vascular tissue is made up of xylem and phloem tissues. Cambium is a meristematic tissue. It is both present in both the young and old stem, but it only develops in the older toot. The pith is also a packing tissue. Strengthening tissue's appear in the cortex and pericycle in older plants.

    Apical meristems bring about the growth in length (height) of the plant. In shoots, they Aldo give rise to branches, leaves and flowers.

      Apical meristems bring about the primary growth (the first growth) of a plant. In many plants, this I'd the only type of growth that occurs. However, in perennial dicotyledons, an increase in the width (or girth) of the plant Aldo occurs. This is known as secondary growth. It is brought about by lateral meristems called cambuim. Vascular cambuim produces new xylem and phloem cells (called secondary xylem and phloem) which are mainly responsible for increase in growth. Cork cambuim produces a thick epidermal layer of water proof cork cells which eventually forms the bark. The bark replaces the epidermis which becomes ruptured during secondary growth.

Note :    Monocotyledons do not have cambuim, and so secondary growth does not occur.

Growth patterns in plants

Growth of plants is studied by measuring dry mass , or length (height) of shoot or root of a plant.

    Annuals exhibit a typical sigmoid growth curve. They show limited growth ad they doe after growing season. In perennials, the growth pattern is a series of sigmoid curves, with each curve representing the growth during one growing season. Perennials are said to show unlimited growth.

    When the stem of a tree showing secondary growth is cut, we can see annual rings or growth rings on the cut surface. each ring represents the growth during one growth season, which is usually a year in temperate countries. In the growth curve, this is seen ad one sigmoid curve. Growth rings can be used to asses the age of a tree.

Factor affecting plant growth

The rage if growth of a plant is affected by

• the availability of nutrients and water ;
• physical factors like humidity , light , temperature and pH of the surrounding; and 
• plant hormones.

     Mineral salts and water are necessary for the healthy rapid  growth of plants. Physical factors affect the metabolic processes and so have an effort on plant growth.

     Plant hormones are also known as plant growth substances. They are produced in small amounts within the plant body. They control growth and growth movement ms by accelerating or inhibiting them. They do this by affecting any or all, of the growth processes, cell division, cell elongation and cell differentiation.

      There are several groups of plant hormones. The main groups that affect growth ate the auxins and gibberellins.

     Auxins are formed mainly at the stem and root apices. If the apices are damaged, growth in length will stop. We can show this by cutting the tip of a young shoot and carrying out the  experiment. The shoot stops crowning when the tip is replaced.

   To show that it is a chemical in the shoot that promotes growth, place a cut shoot tip on an agar block and do the experiment. The growth chemical in the shoot tip diffused into the agar block and promotes growth when the block is placed on the cut end of the shoot.

    Auxins bring about their growth effects by promoting or inhibiting cell elongation in stems and roots. They also stimulate cell division.

• root growth is promoter by very low concentrations of auxins and inhibited by higher concentrations;

• concentrations of auxins which promote maximum root growth have no effect on stem growth;

• stem growth is promoted by high concentrations (100, 000 times the concentration needed for maximum root growth) of auxins; and 

• concentrations of auxins which promote the growth inhibit root growth.

     Thus, we see that different concentration of auxins have different effects on shoots and roots.

    In stems, auxins also inhibit the growth of side branches from axillary buds. If the stem apex or terminal bud is cut off, the axillary buds will grow into side branches. Gardeners cut the stem tips of plants to make them bushy.

     The main effect of gibberellins is to promote cell elongation and bring about growth in the stern. They Also affect cell division and cell differentiation to a certain extent.

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