In this article we will discuss about the two typical dicotyledonous plants which been selected for the study of internal structure of stem with the help of diagrams.
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:
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.
It is the extra-stelar ground tissue lying internal to epidermis.
Cortex is differentiated here into three zones:
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.
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:
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.
2. Cucurbita Stem (Figs. 148 & 149):
Cucurbita stem has wavy margin, thus there are distinct ridges and furrows. Vascular bundles are arranged in two rings, one ring of smaller bundles against the ridges and one ring of larger bundles against the furrows. The central hollow cavity is formed by disintegration of pith.
The following plan of arrangement of tissues is noticed here:
It is single-layered with cuticularised outer walls and many multicellular hairs.
It is differentiated into collenchymatous hypo- dermis, a small parenchymatous region, and single-layered starch sheath with tabular cells attached end to end.
III. The Stele:
It encloses vascular bundles and intra-stelar ground tissues. Vascular bundles are bicollateral. Xylem is in the centre, two strips of cambium on the outer and inner sides of xylem and two patches of phloem on the two sides of cambium. Xylem and phloem have the same elements as in sunflower.
Sieve Tubes and Companion Cells can be clearly observed here:
Intra-stelar tissues. Pericycle is homogeneous, as it is made up of a few layers of sclerenchyma forming a continuous band. All parenchyma cells lying internal to pericycle and in between the bundles are called internal parenchyma.