CBSE Class 10 Answered

Sieve tubes and companion cells are part of phloem. Sieve tubes are thin, elongated tube like structures. They are made up of elongated, thin walled cells, stacked one above the other. Their end walls show numerous pores, hence they are named sieve plates. Mature sieve tubes lack nucleus, but cytoplasm is continuous throughout.

Companion cells are associated with sieve tubes. They nourish the cells of sieve tubes. Companion cells contain dense cytoplasm with large nucleus.