Anatomy of Monocotyledonous Stems
The Monocotyledonous stems are similar to dicotyledonous stems in having an epidermis, a cortex and a stele. The cortex may be well developed and sharply marked off from the stele, or it may be quite narrow and inconspicuous. It is in the structure and arrangement of bundles that monocotyledonous stems differ markedly from dicotyledonous stems.
Stele:
The vascular bundles of monocotyledonous stems, instead of being arranged in a cylinder as in dicotyledonous stems, are usually scattered throughout the stele, including the pith, so that there is no distinction between pith and pith rays. Sometimes the centre of the stele is free from vascular bundles and is occupied by parenchyma cells, which dry up and disappear at an early stage, resulting in a Halloween stem, as in most grasses.
Vascular Bundles:
The vascular bundles of monocotyledonous stems are like those of dicotyledonous stems in consisting of xylem towards the centre of the stele and phloem towards the periphery. The vascular bundles of monocotyledonous stems do not possess a cambium layer, which is found in dicotyledonous stems. This means that monocotyledonous usually do not have secondary thickening. Each bundle remains more or less completely surrounded by a sheath of sclerenchyma cells, the bundle sheath, which is particularly well developed on the sides toward the centre and toward the periphery of the stem. The phloem is made up mostly of sieve tubes and companion cells and the xylem of vessels and wood parenchyma.
The most distinctive and characteristic anatomical features of the monocotyledonous stems are as follows:
- The vascular bundles are many.
- The stele is broken up into bundles. The vascular bundles are lying scattered in the ground tissue of the axis.
- The endodermis is not found. The cortex, pericycle and pith are not differentiated because of the presence of scattered bundles throughout the axis.
- The vascular bundles are collateral and closed. The secondary growth of usual type is lacking, but vestiges of cambial activity in bundles may present in the plant body.
- Leaf trace bundles are numerous. The leaf traces when enter the stem, penetrate deeply. The median traces penetrate bundle somehow or other fuses with other bundle in the due course of time. The anastamoses occur at the nodes.
- Each vascular bundle remains surrounded by a well developed sclerenchymatous sheath.
- The vascular bundles are commonly oval shaped.
- The phloem is represented by sieve tubes and companion cells only. The phloem parenchyma is not found.
- The pith is not marked out.
- Usually sclerenchymatous hypodermis is present.
- Usually epidermal hairs are not present.