In this essay we will discuss about the primary tissues found in the dicotyledonous and monocotyledonous stem of plants.
Essay # 1. Primary Tissues Found in Dicotyledonous Stem:
Anatomically, the stem possesses the dermal tissue system which forms the epidermis, vascular tissue system which forms the vascular bundles and the ground tissue system which includes rest of the tissues found in the stem. All these primary tissues show a great diversity in their structure and arrangement.
A brief account of the tissues found in mature dicot stems are discussed below in basipetal order:
Epidermis is the peripherial protective cell layer in the stem. The compactly arranged cells of this layer are more or less tubular or rectangular with thin primary walls. The outer cell surface of this layer is often covered by a layer of cuticle.
In some cases the stem contains chlorenchyma cells to perform the function of photosynthesis and, in that case, the epidermis is interrupted by the presence of stomata. Glandular and non-glandular hairy outgrowths are formed on the epidermis.
The tissue in-between the epidermis and the stele is the cortex which may contain various types of cells. It may contain thin-walled parenchyma cells only (e.g., Salicornia, Pelargonium etc.) or other types of cells immediately below the epidermis to form the hypodermis.
The cells of this layer differ in origin from the epidermis and structure from the cortical cells below. The hypodermis usually contains peripheral collenchyma cells either as a continuous band or in groups. Groups of collenchyma occur at the ridges of Cucurbita stem.
The photosynthesizing stems contain abundant chloroplastids in the peripheral collenchyma and cortical parenchyma. The hypodermis contains sclereids in Trochodendron, oleoresins in Zingiber, laticifers in Calotropis. The parenchymatous cells of the cortex may contain starch, crystals etc.
It is the inner delimiting layer of cortex. This layer is inconspicuously de-marketed in aerial stems with exception in Piper where casparian strips are also observed. But it is usually prominent in underground stems. The entire vascular cylinder usually remains surrounded by the endodermis but in polystele it encircles the individual vascular bundles as in Nymphaea.
Endodermis in some cases develops casparian strips and Phaseolus stem contains abundant starch to form the starch sheath. Endodermal cells deposit lignin and suberin in addition to cellulose on their transverse and radial walls as strips or bands to form casparian strip or band.
Function of Endodermis:
Endodermis performs the following functions:
1. Fat, starch, protein granules and tannins are accumulated in endodermal cells.
2. The casparian strips of the endodermal cells restrict the movement of water, iron, lead and copper in either direction.
3. The endodermal cells form a barrier for pathogens as it contains large amount of quinones that inhibit the growth of fungi and bacteria.
4. In Cocculus, Paederia foetida stem the cork cambium or phellogen originates from endodermis.
5. Endodermis may behave as meristem termed proendodermis the cells of which divide to form a part of the cortex.
The central core of the vascular and ground tissue systems encircled by the endodermis is the stele. The parenchymatous pith or medulla forms the ground tissue. The pith cells sometimes may be lignified and pitted. The vascular tissues lie in between cortex and pith in dicots as a continuous or split cylinder.
The vascular bundles are conjoint and collateral in which the xylem and phloem lie on the same radii and the phloem remains at the external side. The vascular bundles remain separated by interfascicular parenchyma. In some members of the families Solanaceae, Cucurbitaceae, Apocynaceae etc. phloem is present on both the inner and ouer side of xylem. It is called bicollateral vascular bundle. The internal phloem is also termed mtraxylary phloem.
The phloem strand may also occur as isolated strands in close contact on the inner side of xylem of a vascular bundle (e.g., Calotropis, Strychnos). In dicot stem the collateral and bicollateral bundles are always open with cambium in-between xylem and phloem and all the bundles remain arranged more or less in a ring and lie within a common endodermis to form the monostele structure But in Nymphaea polystele is formed due to enclosure of each vascular strand by an endodermis In some genera, medullary and cortical bundles may also occur.
Essay # 2. Primary Tissues Found in Monocotyledonous Stem:
Monocotyledonous stems also contain the dermal, vascular and ground tissues systems. The dermal tissue system forms the epidermis, vascular tissue system consists of vascular bundles and the ground tissue system consists mainly of parenchyma.
A brief account of the tissues found in mature monocot stems are discussed in basipetal order:
The epidermis of monocot stem is more or less like that of dicots. In contrast to dicot stem, the monocot stem epidermis is usually devoid of glandular and non-glandular trichomes.
b. Ground Tissue:
In monocot stems all the tissues except the epidermis and vascular bundles are described as ground tissue as there is no distinction between cortex and pith.
In certain genera of Liliaceae (e.g., Asparagus) and Amaryllidaceae, however, there is distinction between cortex and stele with distinct endodermis and sclerenchymatous pericycle. The external layers of the ground tissues adjacent to epidermis may be composed of sclerified parenchyma or sclerenchyma.
The vascular bundles are usually scattered throughout the ground tissue to form the atactostele. In Coix, Triticum, Oryza etc. they are, however, arranged more or less in a ring. Individual vascular bundle may be leptocentric or amphivasal (e.g., Dracaena, Aloe etc.) or collateral and closed (e.g., Zea, Asparagus etc.).
In the collateral bundles the metaxylem and protoxylem vessels are arranged in such a way that they appear in the form of the capital ‘Y’ or ‘V’ or ‘U’ in cross section. The large metaxylem vessel elements form the arms of the alphabets and the protoxylem compose the rest.
In some genera, protoxylem cavity or lacuna or canal is formed due to disintegration of the protoxylem. The phloem remains in between the two arms of Y- or V- or U-shaped xylem. A sclerenchymatous bundle sheath usually encircles each vascular bundle.