Generally stem forms the aerial part of the plant. It has characteristic internal organisation.
The stem performs various functions and each function is carried out by a different tissue. The detailed anatomy of stem can be studied under two headings such as dicotyledonous stem and monocotyledonous stem.
Common anatomical features of stem:
(i) Epidermal hairs are mostly multi cellular,
(ii) Cortex is differentiated into zones.
(iii) Vascular bundles are conjoint, collateral, bicollateral or concentric.
(iv) Xylem is end arch.
Common anatomical features of dicot stem:
(i) Cortex is divisible into different zones.
(ii) Hypodermis is usually collenchymatous.
(iii) Vascular bundles are arranged in a ring.
(iv) Vascular bundles are conjoint, collateral open end arch.
(v) A distinct pith is present.
(A) Anatomy of some dicotyledonous stems:
The internal structure of dicotyledonous stems varies in different plants Helianthus and Cucurbita stems described below which are taken as representation of dicot stems.
Internal structure of stem of Helianthas annus (Sun flower):
A thin transverse section of the stem shows following features. The outline of the cross section of stem is circular.
It is the outermost layer. It consists of a single row of closely packed thin parenchymatous cells. A thin layer of cuticle sometimes remains present on the epidermis. Epidermis bears uniseriate multi cellular hairs.
Cortex: It occurs below the epidermis and extends up to the pericycle. It consists of three regions. The outer hypodermis, the middle general cortex and lowermost endodermis.
It occurs just below the epidermis. It consists of a few layers of collenchymatous cells. The cells are thickened at the intercellular spaces. The cells are living and contain chloroplasts.
(ii) General cortex:
It occurs between hypodermis and endodermis. It consists of several layers of thin-walled parenchymatous cells.
This is the inner most layer of cortex. It consists of a single layer of compactly arranged barrel shaped cells. The cells of endodermis contain numerous starch grains. Thus endodermis is referred to as starch sheath.
This zone occurs in between the endodermis and the vascular tissue. It consists of semi lunar patches of sclerenchyma alternating with parenchyma. Each patch of sclerenchyma is associated with the phloem of vascular bundles. Thus it is known as hard bast or bundle cap.
Vascular bundles are arranged in a ring. They are separated from each other by parenchymatous medullar rays. Each vascular bundle is conjoint, collateral and open type. It consists of phloem, combium and xylem.
(i) Phloem: It occurs towards the periphery and consists of sievtubes, companion cells, bast fibers and phloem parenchyma.
(ii) Cambium: It occurs in between phloem and xylem. It consists of 2-3 layers of thin walled, small sized compact and rectangular cells.
(iii) Xylem: It occurs towards the centre and consists of tracheids, vessels, wood parenchyma and wood fibers. Xylem is end arch i.e. protoxylem lies towards the centre, and Meta xylem towards the periphery.
It occurs at the centre. It consists of round, thin walled parenchymatous cells with distinct intercellular spaces.
Pith rays: (Modularly rays)
These are the regions radiating from pith. They occur in between the vascular bundles. They consist of a few layers of radially elongated parenchymatous cells.