Tag: permanent tissues and their types

• Epidermis is the outermost layer of the plant body. It carries out functions as follows:

• Lignin is a substance, which is impermeable to water. It is absent in cork cells.
• Pectin is not present in cork cells. 
• Hemicellulose is permeable to water and gases.
• Cork cells are dead and have cell walls composed of a waxy substance "suberin", which is highly impermeable to gases and water. This substance prevents water loss from the stem. 

Bulliform cells or motor cells are large, bubble-shaped epidermal cells that occur in groups on the upper surface of the leaves of many grasses. Loss of turgor pressure in these cells causes leaves to roll up during water stress. During drought, the loss of moisture through vacuoles induces bulliform cells to cause the leaves of many grass species to close as the two edges of the grass blade fold up toward each other. Once adequate water is available, these cells enlarge and the leaves open again. Folded leaves offer less exposure to sunlight, so they are heated less thus, reducing evaporation and conserving the remaining water in the plant and occur on the leaves of many monocotyledons but are probably best known in grasses. They are thought to play a role in the unfolding of developing leaves and in the rolling and unrolling of mature leaves in response to alternating wet and dry periods. So, motor cells helps in inward rolling of leaves and not in transpiration or guttation. 
Thus, the correct answer is option A.

The periderm is the secondary protective dermal tissue that replaces the epidermis during growth in thickness of stems and roots of gymnosperms and dicotyledons i.e., secondary growth. 

Astrosclerieds is a sclereid having its cell wall drawn out into lobes or arms to form a more or less stellate body. It is found in certain xerophytes. Nymphaea contains large intercellular spaces, which occur in the petiole. These airspaces are surrounded by long columns of parenchyma cells. The columnar cells contain chloroplasts and are presumably photosynthetic. Part of a large astroscleried can be seen cutting across one of the parenchymatous fingers. These sclereids add some mechanical strength to fragile structure. 

Bulliform cells  are so called because of its peculiar bubble shape. It mainly occurs on the upper surface of the leaves present in grasses. Water present in these cells helps to maintain its shape but loss of turgor pressure during the stress allows the leaves to roll up. During drought, the loss of moisture through vacuoles induces bulliform cells to cause the leaves of many grass species to close as the two edges of the grass blade fold up toward each other. Once adequate water is available, these cells enlarge and the leaves open again. 

Two or more tissues together constitutes the tissue system. The activities of the constituent tissues are responsible for the major function, although they are different in structure and origin. All the different type of tissues in a plant that perform the similar basic function, irrespective of their location is known as tissue system. 

The grass leaves are linear in shape and sessile and are rough in texture due to the presence of cork cells in the epidermis and deposition of silica material. The process of deposition of silica material over the epidermis is known as silicification. It checks the transpiration rate and prevents the entry of pathogens and gives stiffness to the lamina.

Histogen theory (By Hanstein 1870) concludes that shoot apical meristem consists three distinct zones.