Anatomy of Dicot and Monocot Roots
different parts of the plants, the various tissues are distributed in
characteristic patterns. This is best understood by studying their internal
structure by cutting sections (transverse or longitudinal or both) of the part
to be studied.
transverse section of the dicot root (Bean) shows the following plan of
arrangement of tissues from the periphery to the centre.
outermost layer of the root is called piliferous
layer or epiblema. It is made up of
single layer of parenchyma cells which are arranged compactly without
intercellular spaces. It is devoid of epidermal pores and cuticle. It possesses
root hairs which are single celled. It absorbs water and mineral salts from the
soil. The chief function of piliferous layer is protection.
consists of only parenchyma cells. These cells are loosely arranged with
intercellular spaces to make gaseous exchange easier. These cells may store
food reserves. The cells are oval or rounded in shape. Sometimes they are
polygonal due to mutual pressure. Though chloroplasts are absent in the
cortical cells, starch grain are stored in them. The cells also possess
leucoplasts. The innermost layer of the cortex is endodermis. Endodermis is
made up of single layer of barrel shaped parenchymatous cells. Stele is
completely surrounded by endodermis. The radial and the inner tangential walls
of endodermal cells are thickened with suberin
and lignin. This thickening was first noted by Robert Casparay in 1965.
So these thickenings are called casparian strips. But these casparian
strips are absent in the endodermis cells which are located opposite the
protoxylem elements. These thin-walled cells without casparian strips are
called passage cells through which
water and mineral salts are conducted from the cortex to the xylem elements.
Water cannot pass through other endodermal cells due to the presence of
tissues present inside endodermis comprise the stele. It includes pericycle and
is generally a single layer of parenchymatous cells found inner to the endodermis.
It is the outermost layer of the stele. Lateral roots originate from the
pericycle. Thus, the lateral roots are endogenous in origin.
tissues are in radial arrangement.
The tissue by which xylem and phloem are separated is called conjunctive tissue. In bean, the
conjuctive tissue is composed of parenchyma tissue. Xylem is in exarch condition. The number of protoxylem points is four and so the xylem is called tetrach. Each phloem patch consists of sieve tubes, companion cells and phloem parenchyma. Metaxylem
vessels are generally polygonal in shape. But in monocot roots they are