BIOLOGY CH18 Organization of Tissues — Science STD 9 — Question

Phloem is a complex tissue and consists of the following elements :(i) Sieve elements(ii) Companion cells(iii) Phloem fibres(iv) Phloem parenchyma(i) Sieve elements :

• The conducting elements of phloem are collectively called as Sieve elements.
• Sieve tubes are elongated, tube-like slender cells placed end to end. The transverse walls at the ends are perforated and are known as sieve plates.
• The main function of sieve tubes is translocation of food, from leaves to the storage organs of the plants.

(ii) Companion cells : These are elongated cells attached to the lateral wall of the sieve tubes. A companion cell may be equal in length to the accompanying sieve tube element or the mother cell may be divided transversely forming a series of companion cells.(iii) Phloem parenchyma : The phloem parenchyma are living cells which have cytoplasm and nucleus. Their function is to store food materials.(iv) Phloem fibers : Sclerenchymatous cells associated with primary and secondary phloem are commonly called phloem fibers. These cells are elongated, lignified and provide mechanical strength to the plant body.