Tag: internal structure of monocot stem

In a monocotyledenous stem the vascular bundles are not arranged in a circle but are usually scattered throughout the ground tissue. There is no clearly defined pith as there is in many dicotyledonous species. 

Secondary growth is increase in the circumference / girth of the plant organs due to the formation of secondary tissues in stelar and extra stelar regions. Normally secondary growth takes place in roots and stem of dicotyledons and gymnosperms. Due to lack of cambium in monocotyledons, secondary growth is absent. But exceptionally, secondary growth takes place in some monocotyledons, such as palm, Yucca, Dracaena etc. 

In monocot stem vascular bundles are found irregularly scattered in the ground tissue. Towards the periphery, the bundles are smaller in size, while towards the centre, they are larger in size. The smaller bundles are younger, while the larger ones are older. Hence, the arrangement is described as centrifugal. 

Atactostele is a type of eustele, found in monocots, in which the vascular tissue in the stem exists as scattered bundles. Most seed plant stems possess a vascular arrangement, which has been interpreted as a derived siphonostele and is called as eustele. There is also a variant on the eustele found in monocots, like maize and rye. The variation has numerous scattered bundles in the stem and is called as an atactostele.

1. Haplostele consist of smooth ore of xylem which is surrounded by a phloem ring.ex-Seleginella
2. Actinostele consist of star shape xylem core with many radiating arms.ex-Lycopodium
3. Plectostele consist of xylem core broken into a number of parallel plates.ex-Lycopodium clavatum
4. Siphonostele is a protostele with central pith and considered to be the most advance stele.ex-Equisetum.

In the monocots, the vascular bundles are of the collateral and closed type. The closed vascular bundles lack the cambium and cannot show secondary growth. In the stem, vascular bundles are found scattered in the ground tissue. They are more in number at the periphery as compared to the centre. In the monocot roots, the vascular bundles are closed type an found in the stele.

In monocot stem, all the vascular bundles, the xylem and phloem are in the same line; cambium is absent; phloem is external to xylem; protoxylem faces the centre. Hence, the vascular bundles are described as conjoint, collateral, endarch and closed.  

The region present below the epidermis in monocot stem is hypodermis. It is composed with sclerenchyma. Sclerenchyma is dead mechanical tissue. It provides mechanical strength to the stem.

So, the correct option is 'Sclerenchyma’.