Question

The protostele in which xylem core is smooth and rounded is
A. Haplostele
B. Actinostele
C. Plectostele
D. Siphonostele

Answer 1

The most primitive type of stele is known as protostele in which vascular cylinder of solid core of xylem is surrounded by phloem, pericycle and endodermis. A central and smooth core of xylem is found which is surrounded by phloem. In actinostele, xylem appears as stellate or star shaped. Plates are formed in the plectostele. Central pith is present in siphonostele.

Complete answer:
The concept of stele was developed by French botanists P.E.L. van Tieghem and H. Doulton in the 19th century. Jeffrey in 1897 suggested the term protostele. Two types of protosteles suggested by Berbner(1902). The central core of the stem and root of a vascular plant, consisting of vascular tissue (xylem and phloem) and it is also associated with other supporting tissue.
Protostele is a primitive and simple type of stele. Xylem is surrounded by phloem, pericycle and endodermis which form the vascular tissues. In adult stems of many living pteridophytes it is present permanently.it is present in vascular plants. Four types of protosteles are: haplostele, actinostele, plectostele, and mixed protostele.
Haplostele consists of a cylindrical core of xylem which is surrounded by a phloem ring. Centrarch is present in the middle and endodermis surrounds the stele. It involves the members of rhyniophyta grade, such as Rhynia.
In the action the core is lobed and fluted. This type of arrangement is mostly found in many species of club moss. Vascular bundles are of exarch type. Many patches of protoxylem are present at the tip of lobes of metaxylem. Xylem shows pointed projections in cross section or ridges in three dimensions with phloem in furrows.
In plectostele, plate appears in the transverse section surrounded by phloem tissue.phloem appears in between the plates. Plectostele are derived from actinosteles. Some modern club mosses consist of plectosteles.
Siphonostele consists of a pith that is the region of ground tissue. They have an open xylem core. They can be ectophloic or amphiphloic. Eusteles are derived from siphonostele.
From the above information we can conclude that haplosteles have a smooth and rounded xylem core.

So, option A is the correct answer.

Note: Monocots have discontinuous vascular systems for example grasses which contain scattered vascular bundles. Dicots have continuous vascular bundles which surround the central pith for example in roses. Many roots have simple actinosteles, as they are rich in diarch, triarch, or tetrarch. Amphiphloic and ectophloic siphonostele are found in ferns.