Page 7 - LN-ANATOMY OF FLOWERING PLANT

The xylem parenchyma can be sub-divided into two types Primary Xylem The xylem
differentiating in the primary plant body is the primary xylem. The primary source of
this xylem is the procambium. The primary xylem is of two types, i.e., protoxylem and
metaxylem.
The first formed primary xylem elements are called protoxylem.The latter formed
primary xylem is called 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.
Secondary Xylem is composed of tracheary elements, rays, fibres and interspersed
axial parenchyma cells. The cell formed toward inside of cambia are called
secondary xylem or wood. The primary function of secondary xylem is to provide
mechanical support to plants.
ii. Phloem
Phloem (Gk. Phbis—bark) is a food conducting complex permanent tissue. The term
‘phloem’ was coined by Nageli (1958). In angiosperms, it is also called bast. In
gymnosperms, albuminous cells and sieve cells are present. The first formed
primary phloem consists of narrow sieve tubes called protophloem and the latter
formed phloem has bigger sieve tubes called metaphloem.

It consists of four types of cellular components,
(a) Sieve Elements The sieve tube elements are long, tube-like structures arranged
longitudinally and are associated with the companion cells. Their end walls are
perforated in a sieve-like manner to form the sieve plates. A mature sieve element
possesses a peripheral cytoplasm and a large vacuole, but lacks a nucleus, Golgi
body and most cytosol.
Sieve elements are of following two types
• Sieve cell It is a special kind of cell which posses sieve areas in its lateral walls.
There is no specialised plate in it. Sieve cells are usually found in pteridophytes and
gymnosperms.
• Sieve tube members In this type, the sieve areas are localised on its end walls.
Sieve tube members are placed one above the other forming a continuations tube
called sieve tube. The end walls are perforated (sieve pores) like a sieve. These are
found in angiosperms.
The uniqueness of the sieve tube is that although without nucleus, it is living and the
nucleus of the companion cell controls its functions.