In stem there exist three tissue systems — dermal, vascular and ground tissue, which respectively consist of epidermis, vascular bundle and all tissues except the former two. A great diversity is observed in the arrangement and structure of primary tissues, a brief account of them that commonly occur in mature stems of dicot is mentioned below beginning from the periphery of stem.
Externally, the stem is bounded by epidermis. To mention briefly it is the peripheral layer of stem and is uniseriate. The cells are more or less tabular or rectangular with thin primary walls. The cells are usually compactly set and contain living protoplast.
Cutin is often deposited on the outer surface of cells to form cuticle. In stems, which carry on photosynthesis, the continuity of epidermis is interrupted by the presence of stomata. Glandular and nonglandular hairs are also observed on the epidermis.
The cylindrical region that occurs between epidermis and vascular cylinder is the cortex.. Various types of cell are observed in this region. It may consist of thin walled parenchyma cells only (ex. Salicornia, Pelargonium etc.). In many stems the peripheral layers of cortex, immediately below the epidermis, may consist of other types of cell.
This layer is usually termed as hypodermis and its constituent cells differ in origin from the epidermis and structure from the cortical cells below. Usually the peripheral layer contains collenchyma that occurs either as a continuous band or discontinuously in groups or as isolated strips. Patches of collenchyma occur at the projecting ridges of stem.
The stems, which carry on photosynthesis, contain abundant chloroplastids in the peripheral collenchyma and cortical parenchyma, and these cells are known as chlorenchyma. Sclereids (ex. Trochodendron), oleoresin cells (ex. Zingiber), laticifers (ex. Calotropis) etc. may also occur here. The cortical parenchyma may contain starch, crystals etc.
It is the cylindrical layer of cells that delimit cortex from the stele. This layer is inconspicuous in aerial stems. It is prominent in underground stems and sometimes in aerial stem like Piper where casparian strips are also observed. It usually surrounds the entire vascular cylinder and in exceptional case it encircles individual vascular strand where the stem is polystele (ex. Nympliaea).
The innermost layer of cortical cells of many dicotyledonous stems (ex. Phaseolus) contains abundant starch. This layer is called starch sheath; Starch sheath, in some stems, may be transformed into endodermis and develops casparian strips.
Endodermis consists of a layer of living cells that are compactly set and have characteristic wall structure. Lignin and suberin, in addition to cellulose, deposit on the transverse and radial walls of endodermal cells as strips or bands. This layer is called casparian strip or band. A mature endodermal cell contains suberin, cellulose and often lignin on the entire inner surface of walls.
Function of Endodermis:
(1) It accumulates fat, starch, protein granules, tannins and carbohydrates.
(2) The suberized cell wall of endodermis acts as a barrier to the movement of iron, lead and copper either in or out of vascular tissues.
(3) It excretes water into trachea as it possesses higher hydrostatic pressure than the neighbouring cortical cells.
(4) It is impermeable to gaseous diffusion and so it acts as a barrier to gaseous exchange.
(5) It controls the conduction between the central cylinder and cortex in rhizomes.
(6) It forms a barrier of pathogens as it contains large amount of benzoquinones, naphthoquinones and anthroquinones that inhibit the growth of fungi and bacteria.
(7) In Cocculus, Paederia foetida stem the cork cambium or phellogen originates from endodermis.
(8) Endodermis may behave as meristem termed proendodermis that divides to form a part of the cortex.
The tissues that lie internal to endodermis compose the stele. It includes the vascular and ground tissue systems. The ground tissue consists of pith or medulla. The pith is usually parenchymatous and sometimes may be lignified and pitted.
In dicotyledonous stems, the vascular tissues lie in between cortex and pith. It occurs in the form of a continuous or split cylinder. The phloem is situated on the peripheral or external side. In contrast to root, xylem and phloem of stem lie on the same radii. Each strand of the split vascular cylinder is designated as vascular bundle that is separated by interfascicular parenchyma.
A vascular bundle with external phloem only is termed as collateral bundle. In some plants belonging to families Solanaceae, Cucurbitaceae, Apocynaceae etc. phloem is also present on the inner side of xylem.
To distinguish it from external phloem, the inner phloem is termed as internal or intraxylary phloem. This phloem occurs as isolated strands either on the peripheral margin of pith (ex. Calotropis, Strychnos) or in close contact on the inner side of xylem of a vascular bundle.
In latter case the vascular bundle with both internal and external phloem is termed as bicollateral vascular bundle (ex. Cucurbita). In dicotyledonous stem the collateral and bicollateral bundles are always open, i.e. cambium is present in between xylem and phloem.
Usually the stele of dicotyledonous stem is monostele, i.e. the vascular bundles are arranged more or less in a ring and lie within a common endodermis. But variations occur when each vascular strand becomes completely enclosed by an endodermis. Ex. Nymphaea (Nymphaeaceae). In some genera medullary and cortical bundles occur.