A) pectin
B) callus
C) suberin
D) lignin
Correct Answer: B
Solution :
At the end of growing season the sieve plate is surrounded by a highly referangible substance called callus (formed by a carbohydrate called callose). When callus is formed the pores are blocked and so the transport of food material is retarded. At the advance of spring this substance dissolve and the pores open again. Lignin is an organic compound of high carbon content. The impregnation with lignin usually starts in the intercellular lamella and then spreads centripetally through the primary and secondary walls e.g., xylem cells. Pectic substances (protopectin, pectin, pectic acid) constitute the intercellular substances that binds together the wall of individual cells. Suberin occurs in association with cellulose in cork cells of periderm, endodermis and exodermis of many plants.
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