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A Handbook of Biology

They have highly thickened walls and obliterated central

lumens. They are septate or aseptate.

XYLEM FIBRES

Living and thin-walled cells with cellulosic cell walls. They

store food materials (starch or fat) and other substances

like tannins. Radial conduction of water takes place by

the ray parenchymatous cells.

XYLEM PARENCHYMA

Primary xylem is of 2 types:

METAXYLEM

PROTOXYLEM

The first formed primary xylem.

The later formed primary xylem.

Metaxylem:

In stems, the protoxylem lies towards the centre (pith) and the metaxylem

lies towards the periphery of the organ. This type of primary xylem is called

endarch.

In roots, the protoxylem lies towards periphery and metaxylem lies towards

the centre. Such arrangement of primary xylem is called exarch.

B. Phloem (Bast)

It transports food materials from leaves to other parts. In angiosperms,

phloem is composed of sieve tube elements, companion cells, phloem

parenchyma & phloem fibres. Gymnosperms and pteridophytes have

albuminous cells and sieve cells. They lack sieve tubes and companion cells.

(NEET 2019)