Question

You are given a fairly old piece of dicot stem and a dicot root. Which of the following anatomical structures will you use to distinguish between the two?
A. Secondary xylem
B. Secondary phloem
C. Protoxylem
D. Cortical cells

Answer 1

Meristematic cambium cells found in a layer just inside the bark of trees and shrubs create xylem and phloem tissues. The cambium layer in the dicot stems gives rise to phloem cells on the outside and xylem cells on the inside. All the tissue is called bark from the cambium layer outward, while all the tissue is wood within the cambium layer to the middle of the tree. In plant roots and stems, Xylem tissue conducts water and mineral nutrients upward from the soil.

Complete answer:
Secondary xylem: Xylem formation begins when primary xylem is formed by the actively dividing cells of the rising root and shoot tips. Secondary xylem is the main part of a mature stem or root in woody plants and is shaped as the plant grows in girth and constructs a ring of new xylem around the original primary xylem tissues. The primary xylem cells die as this occurs and lose their conducting ability, forming a hard skeleton that only serves to protect the plant.
Secondary phloem: During secondary development, the secondary phloem is a type of phloem that forms from the vascular cambium. The growth of girth in plants, in particular trees, is responsible for secondary growth. In this form of development, the vascular cambium is the meristematic tissue involved. Some of the vascular cambium formed cells can differentiate into secondary phloems.
Protoxylem: The first xylem that grows is the protoxylem. It is made up of narrow boats. The vessels are made up of cells which are comparatively smaller than metaxylem cells. Nevertheless, there are more parenchyma cells in the protoxylem than metaxylem. The protoxylem in stems in young plants has very little secondary wall material. It is organised in the stem towards the middle, while it is in the root towards the periphery. The location of the protoxylem separates the old dicot root from the dicot stem. The protoxylem in the stem is towards the pith and in the root, towards the periphery.
Cortical cells: Cortical cells are spindle shaped and parallel to the axis of the fibre are oriented. There are several types of cortical cells: ortho and para, with slightly different physical and chemical properties, and the less common intermediate mesocortical cells, with an intermediate structure between ortho and para.

Hence the correct answer is option C.

Note:
The arrangement of the xylem and phloem tissue depends on whether the plant is a flowering plant or a gymnosperm polycot, like dicots and monocots. In plant stems, the water-conducting xylem tissue is actually made up of dead cells. Currently, wood is simply dead cells of xylem that have dried out.