In this article we will discuss about the internal structure of monocot stem with reference to a typical monocotyledon, the maize stem, and carina scape.
1. Maize Stems (Figs. 150 & 151):
It is as usual single-layered with cuticularised outer walls.
II. Ground Tissues:
Next to epidermis there are two or three layers of sclerenchyma forming the hypodermis. Ground tissues internal to hypodermis are all parenchymatous in nature showing the primary body no differentiation into cortex, starch sheath, etc. They are simply called ground tissue.
Bundles are numerous and scattered in the ground tissue, more crowded towards the periphery than towards the centre. The bundles are collateral closed. That is why monocotyledons usually do not grow in thickness. Xylem has the usual elements. Two metaxylem vessels lie outwards and two protoxylem vessels towards the centre.
Their arrangement is more or less like the letter Y. Beneath the lowest protoxylem there is a cavity called protoxylem cavity. Phloem is much smaller and is composed of only sieve tubes and companion cells, phloem parenchyma being absent. The whole bundle remains surrounded by sclerenchymatous tissue forming what is called the bundle sheath.
2. Flowering Stem or the Scape of Canna (Fig. 152):
It also shows typical monocotyledonous structure, though a bit different from the maize stem.
I. Epidermis is the single-layered protective zone with cuticularised outer walls.
II. Ground tissues. Next to epidermis there are a few layers of parenchyma cells forming the cortex which is immediately followed by a band of chloroplast-containing cells called chlorophyllous tissues. Patches of sclerenchyma remain internally attached to chlorophyllous tissue here and there. The remaining portion is parenchymatous ground tissue enclosing the scattered vascular bundles.
III. Vascular bundles are collateral closed. Each of them has small amount of xylem and phloem, with only sieve tubes and companion cells. Sclerenchyma cells do not form a sheath surrounding the whole bundle, as in the maize stem, but remain in two patches on the outer and inner sides of the bundle. The patch on the outer side is larger forming something like a cap and that on the inner side is much smaller.
It has been noticed that all these stems have protective epidermis, ground tissues and conducting tissue with xylem and phloem. Xylem is typically endarch in the stems.