Page 14 - LN-ANATOMY OF FLOWERING PLANT

free passage of water and minerals from cortex into the xylem. These are called
passage cells.
4. Stele
All tissues on the innerside of the endodermis such as pericycle, vascular strand and
pith constitute the stele.
i. Pericycle The next to endodermis lies a layer of thick-walled parenchymatous cells
referred to as pericycle. The initiation of lateral roots and vascular cambium during
the secondary growth takes place in these cells.
ii. Vascular Strand The vascular strand consists of separate bundles of xylem and
phloem arranged alternately inner to the pericycle. Hence, the xylem and phloem
bundles are equal in number and lie on different radii. Such vascular bundles are
called radial bundles.
On the basis of number of xylem bundles, the root may be diarch (with two xylem
bundles), triarch, tetarch, pentarch and polyarch (with more than five xylem bundles).
When the protoxylem is towards the periphery and the later formed xylem
(metaxylem) is towards the centre of the root. This kind of xylem condition is called
exarch and is characteristic of root.
The phloem and xylem bundles are separated from each other by one or more layers
of small thin-walled cells called conjuctive tissue.
Later, it becomes meristematic and forms vascular cambium. The phloem tissues
conducts organic food from leaf to the other parts of the plant. Secondary, growth
occurs in dicot roots.
iii. Pith It is generally absent in dicot roots. If present, it is small. It consists of
parenchyma cells that store food and waste products.
Features for Identification of Dicotyledonous Root
Dicotyledonous root can be easily identified with the followingfeatures
(i) Presence of root hairs.
(ii) Endodermis with casparian strips.
(iii) Absence of pith.
(iv) Radial bundles less than eight.
(v) Presence of exarch xylem.
II.Ionocotyledonous Root
This can be inferred from the following structures given below
1 Epiblema
It is the outermost, thin-walled, compactly arranged layer of cells. Some of the cells
give rise to root hair. The root hair are unicellular and lie in contact with soil water.
Both epiblema and root hair are devoid of cuticle. These helps in absorption of water
and minerals. In older parts the epiblema is shed or impervious.
2 Cortex
It is a broad zone of parenchyma cells. The cells are thin-walled and enclose
intercellular spaces. They normally store food. The cortex provides for radial
movement of water and minerals from epiblema to the root interior.