question_answer 77)

  Give reasons for (a) Meristematic cells have a prominent nucleus and dense cytoplasm but they lack vacuole, (b) Intercellular spaces are absent in sclerenchymatous tissues. (c) We get a crunchy and granular -reeling, when we chew pear fruit. (d) Branches of a tree move and bend freely in high wind velocity. (e) It is difficult to pull out the husk of a coconut tree.

Answer:

  (a) Meristematic cells are the continuously dividing cells of the plant body i.e., they remain metabolically active and as we know that vacuoles serve the purpose of storage in plant cells. Therefore meristematic cells do not require vacuole. (b) Sclerenchyma tissue are dead simple permanent tissue of the plant. The cells of sclerenchyma are closely packed without intercellular spaces, like tiles in mosaic floor so that, it can provide the strength, rigidity, flexibility and elasticity to the plant to withstand various strains. (c) The get a crunchy and granular feeling when we chew pear fruit due to the presence of sclerenchyma tissue. The cell walls of sclerenchyma are greatly thickened due to the presence of lignin. Lignin is a complex polymer with high tensile strength i.e., it does not breaks easily on stretching and a high compressional strength, it does not buckle easily. (d) Collenchyma cells are characterized by the deposition of extra cellulose at corners of the cell. In collenchyma intercellular spaces are absent and cells are elongated. It is a mechanical tissue which provides support and elasticity to the branches of a tree so that, they move and bend freely in high wind velocity. (e) It is difficult to pull out the husk of a coconut tree due to the presence of sclerenchyma tissue. The cells of sclerenchyma tissue are closely packed without intercellular spaces, a middle lamella exists between them. Middle lamella is a thin layer of connecting substance containing pectin, lignin and protein.