DICOTYLEDONOUS STEM
The transverse section (TS) of a typical young
dicotyledonous stem shows the following areas
1. Epidermis
The outermost protective layer of the stem is called
epidermis. It is covered with a thin-layer of cuticle and may
bear trichomes and a few stomata. The cuticle protects the
tissues from injury as well as diseases from the entry of
fungal spores and bacteria. It also helps to prevent loss of
water.
2. Cortex
This layer lies just below the epidermis and extends till
endodermis. Its various parts are hypodermis, general cortex
and endodermis.
i. Hypodermis
It is just below the epidermis consisting of collenchymatous
cells. The cells contain chloroplasts. It provides mechanical
strength to the stem.
ii. General Cortex
It is located just below the hypodermis and consists of a few
layers of parenchymatous cells. These cells are thin-walled
have big intercellular spaces and may contain chloroplasts.
iii. Endodermis
It lies just beneath the general cortex in the form of single
layer of barrel-shaped cells surrounding the stele. It is the
innermost layer of cortex. In sunflower, it contains starch,
hence is called starch sheath.
3. Pericycle
It exists between the endodermis and the vascular bundles.
The cells are sclerenchymatous with lignified cell walls and
a few parenchymatous cells dispersed in between. Each
patch is associated with phloem of the vascular bundle and
is called the hard bast.
4. Vascular Strand
The vascular strand consists of many vascular bundles,
arranged in the form of a ring around a central pith and
inner to pericycle.
Each vascular bundle consists of phloem (on the outside),
xylem (towards the inner side) with a strip of cambium, in
between the two.
The vascular bundles are thus, conjoint (i.e., consists of
both xylem and phloem), collateral (i.e., phloem and xylem
are on the same radius), open (i.e., a strip of cambium
present between the two) and with endarch protoxylem.
5. Medullary or Pith Rays
These are non-vascular areas present in between the
vascular bundles. The medullary rays connect pith with
pericycle and cortex. Cells are larger than those of cortex.
The medullary rays take part in radial conduction of
materials, i.e., food, water, gases, etc.
6. Pith or Medulla
It consists of the central part of the stem. It consists of
rounded, oval or polygonal parenchymatous cells.
Intercellular spaces are absent. The cells store food materials
and waste products.
Features for Identification
of Dicotyledonous Stem
Dicotyledonous stem can be easily identified with the
following features
(i) Occurrence of multicellular hair over epidermis.
(ii) Collenchymatous hypodermis.
(iii) Presence of bundle caps or sclerenchymatous
pericycle over vascular bundles.
(iv) Endarch xylem.