BOTANY Higher Secondary Second Year - Textbooks Online

Phloem
Like xylem, phloem is also a complex tissue. It conducts food
materials to various parts of the plant. The phloem elements which are
formed from the procambium of apical meristem are called primary
phloem. The phloem elements which are produced by the vascular cambium
are called secondary phloem. The primary phloem elements that develop
first from the procambium are smaller in size called the protophloem,
whereas those develop later are larger in size called metaphloem. The
protophloem is short lived. It is crushed by the developing metaphloem.
Phloem
parenchyma
Companion
cell
Sieve tube
Fig. 2.7. Phloem tissue
Phloem is composed of four kinds of cells: sieve elements, companion
cells,phloem parenchyma and phloem fibres. Companion cells are present
only in angiosperms. Companion cells are absent in pteridophytes and
gymnosperms. Phloem fibres are absent in the primary phloem of most of
the angiosperms. But they are usually present in the secondary phloem.
Sieve elements
Sieve elements are the conducting elements of the phloem. They have
thick primary walls. Their end walls are transverse or oblique. The end
wall contains a number of pores and it looks like a sieve. So it is called a
sieve plate.The sieve elements are arranged one above the other and form
vertical sieve tubes. In matured sieve tube, nucleus is absent. It contains
76
Callose
Slime body
Cytoplasmic
strands
Sieve plate
Lignified
wall
Lumen
Phloem fibre (bast fibre)