Question

draw the section cutting of Dicot stem and explain it briefly​

Answer 1

Answer:

1. Young Sunflower Stems (Figs. 146 & 147):

If a thin and uniform transverse section is taken from a young sunflower stem and observed under the microscope, the following plan in the arrangement of tissues will be noticed:

. Epidermis:

It is the single-layered outermost skin com­posed of tabular parenchyma cells which are compactly arranged without having intercellular spaces. Outer walls are cuticularised. Stomata may be present here and there on the epidermis. Many multicellular hairy outgrowths are noticed

II. Cortex:

It is the extra-stelar ground tissue lying internal to epidermis.

Cortex is differentiated here into three zones:

(a) Hypodermis:

It is composed of a few layers of collenchymatous cells with thickened corners forming a band just beneath the epidermis. They are meant for mechanical support.

(b) Parenchyma

Next to collenchymatous hypodermis many layers of parenchyma cells with intercellular spaces are found. They often enclose a number of glands.

(c) Starch-Sheath or Endodermis:

It is the last layer of cortex. It is one-layered with barrel-shaped closely fitted cells forming a wavy band, delimiting the central cylinder or stele.

These cells contain abundant starch grains, hence called starch sheath.

III. The Central Cylinder or Stele:

It encloses many vas­cular bundles arranged in form of a distinct ring and the intra-stelar ground tissues.

(a) Vascular Bundles:

These are placed more towards the epidermis than towards the centre. Bundles are collateral and open. Xylem with its component parts, vessels, tracheids, wood-fibres and wood parenchyma, lies towards the centre.

Protoxylem vessels with smaller cavities and annular or spiral thickenings are always placed towards centre, and metaxylem vessels, with wider cavities and reticulate, scalariforrn or pitted thickening, towards, the epi­dermis.

This arrangement, typical of stems, is called endarch. Phloem with sieve tubes, companion cells and phloem parenchyma lies outside or towards the circumference. A strip of lateral meris­tem, called cambium, is present between xylem and phloem.

(b) Intra-Stelar Ground Tissues:

These are differentiated into three zones:

(i) Pericycle:

Next to endodermis there is a many- layered region, called pericycle, lying outside the bundles. Peri­cycle in the sunflower stem is made up of two types of cells. Against every vascular bundle there is a patch of sclerenchyma forming something like a cap on the bundle. These patches are called bundle caps or hard bast. In between the two bundles, pericycle is parenchymatous. Thus the pericycle is heterogeneous.

(iii) Pith or Medulla:

It is the large central portion of stem occupied by thin-walled colourless parenchyma cells with lot of intercellular spaces.

(iii) Medullary Rays:

Parenchymatous cells present in bet­ween every two vascular bundles look like rays radiating from the pith; they are called medullary rays.