Q. What are the function of apical, lateral and intercalary meristem?

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Answer to your question is easily available in Chapter 6, lesson 1. Kindly go through the lesson and if doubt still persists then do get back to us.

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Meristems and Simple Plant Growth

Plants have meristematic tissue in several locations. Both roots and shoots have meristematic tissue at their tips called apical meristems that are responsible for the lengthening of roots and shoots. The shoot apical meristem is formed during embryonic development, but after germination gives rise to the stem, leaves, and flowers. The root apical meristem is also formed during development, but during germination gives rise to the root system. Cell division and cell elongation in the apical meristem is called primary growth and results in an increase in plant height and root length. Increasing root length enables the plant to tap into the water and mineral resources of a new region or layer of soil. Increasing shoot length makes the plant taller, thus allowing it better access to sunlight for photosynthesis.

Many types of plants also increase the diameter of their roots and stems throughout their lifetime. This type of growth is called secondary growth and is the product of lateral meristem. Lateral meristem is called the vascular cambium in many of the plants in which it is found. Secondary growth gives a plant added stability that allows for the plant to grow taller. Lastly, some plants have intercalary meristem. These are areas of plants that help in the regeneration of parts of the plant that have been damaged by predators or the environment. Intercalary meristems produce growth at the base of grass blades, for instance.

Meristem tissue is not autonomous. Throughout the life of the plant, the rate of cell division and cell elongation in the meristems is regulated by plant hormones . For example, giberellins stimulate cell division in shoot apical meristem, causing the plant to grow taller. These hormones also cause cell elongation in intercalary meristem of grasses. Cytokinin and auxin are also important growth regulators. Auxin stimulates growth by inducing cell elongation, while cytokinins are thought to stimulate both cell division and cell elongation.

 

Meristem locations, with cross-sections of vascular tissues.
Meristem locations, with cross-sections of vascular tissues.

Apical Meristems and Pattern Formation

As the source for all new cells of the growing plant, the meristem plays an important role in the formation of new organs and in the correct placement of those organs within the plant body. The process by which this organization happens is called pattern formation and, in plants, is directed by the meristem. To accomplish this task, meristematic cells must be able to interpret their position in the plant and establish a certain fate.

For example, during the development of a new leaf, the dividing cells of the meristem must differentiate into several different functional types of epidermal cells and parenchyma cells. However, they do not need to differentiate into reproductive cells like those found in a flower. How is it that meristematic cells "know" what to become? The actively dividing cells of the apical meristem use positional cues such as hormones and cell-cell interactions as guides during differentiation. Moreover, these positional cues result in the activation of certain genes and the inactivation of other genes in a set of cells, thus initiating their specific differentiation pattern based on their spatial location in the plant. The specific genes that are initially activated in meristem cells during this process are called homeotic genes. These genes encode a family of transcription factors that, once activated, will determine the fate of a cell by activating and inactivating a whole host of other genes.

One mechanism of differential gene expression (the activation and inactivation of genes during differentiation and organ development) is binding of plant hormones to the developing cell's surface. Hormones such as cytokinins have been shown to affect ribonucleic acid (RNA) transcription and translation . It is thought that the presence of both cytokinins and another class of hormones, called auxins, are important for proper root and shoot development. In the laboratory, if a set of undifferentiated meristem cells are grown in culture, they will not develop into a plant embryo unless they are stimulated with auxin and cytokinin. 

Apical -it is present at d growing regions.eg.root n sshoot tips.it increases d length of d plant.

intercalary -present at d base of leaves n internodeit also increase length of plants

lateral -present in the cambium,increases d thickness of plants

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Apical Meristem: Apical meristems are located near the tips of roots and shoots. A typical meristem cell is relatively small, cuboid,with a thin cellulose cell walland dense cytoplasmic contents.When the cell divided by mitosis one cell remain in the neristemwhile the other increase  in size and differentiates to become a part of the permanent plant body. Apical meristem is responsible for primary growth and thus responsible for the increase in length of the plant. The apical initials may occur in one or more tiers.

Lateral Meristem: Lateral meristems are laterally situated in older parts of the plant parallel with the long axis of the organs. They occur in thin layers. It is responsible for secondary growth. Vascular cambium gives rise to secondary vascular tissue. Phellogen gives rise to the periderm which replaces the epidermis and includes cork. It is responsible for the increase in girth of the plants.

Intercalary Meristems: Intercalary meristems are located between the regions of permanent tissue, at the nodes of many cotyledons, such as at the base of grass leaves.  It allows an increase of length. The lateral meristems gives rise to the tissues responsible for this thickening of stems and roots. The main function of this type of meristem is to increase the length of the organin which they occur. They are short lived and gradually merge with the surrounding tissues and become permanent.

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