CDS (II) 2024 General Knowledge · Question 7 of 120

Correct — D, Endodermis. NCERT's chapter on plant anatomy defines the bundle in one line — the xylem and phloem together constitute vascular bundles — and adds that in a dicot stem a cambium lies between the phloem and the xylem, which is what makes such a bundle an open one able to produce secondary tissue. So all three of the first options are genuine components of a vascular bundle. The endodermis is not. The same chapter places it elsewhere: the innermost layer of the cortex is called endodermis, a single ring of barrel-shaped cells with waxy suberin laid down in the wall as casparian strips. It is a boundary layer that regulates what passes from the cortex inward, and everything inside it — pericycle, vascular bundles and pith — makes up the stele. The endodermis therefore encloses the bundles; it is not one of them.

• (a)Phloem — Phloem is one of the two tissues that define a vascular bundle. It carries the food made in the leaves to the rest of the plant, and in a conjoint bundle it usually lies on the outer side of the xylem.
• (b)Xylem — Xylem is the other defining tissue of the bundle, carrying water and dissolved minerals upward from the roots. Xylem and phloem together are what the word vascular refers to.
• (c)Cambium — Cambium sits inside the bundle, between the xylem and the phloem, in dicot stems. Its presence makes the bundle open and capable of secondary growth; monocot bundles lack it and are called closed. Either way, when it is present it is part of the bundle.

A plant body is organised into three tissue systems: the epidermal system on the outside, the vascular system of xylem and phloem, and the ground tissue that is everything else. In a young root or stem seen in cross-section the layers run from outside inward — epidermis, cortex, endodermis, pericycle, then the vascular bundles around a central pith. Everything inside the endodermis is collectively called the stele.

The item is really a test of where the boundary of a bundle is drawn. Two tissues make the bundle, a third may sit within it, and the fourth option names a layer that lies outside it. The most useful thing to carry away is the type vocabulary that follows from the same paragraph: a bundle with cambium is open and can thicken over the years, which is why a dicot tree grows in girth; a bundle without cambium is closed, which is why a palm or a bamboo does not thicken the same way. Bundles are also described as radial when xylem and phloem lie on alternate radii, as in roots, and conjoint when the two sit on the same radius, as in stems and leaves.

• NCERT states that the xylem and phloem together constitute vascular bundles.
• Cambium between the phloem and xylem makes a bundle open, capable of forming secondary xylem and phloem; monocot bundles lack cambium and are called closed.
• The endodermis is the innermost layer of the cortex, a single ring of cells whose walls carry waxy suberin as casparian strips.
• Next to the endodermis lies the pericycle, where lateral roots and the vascular cambium of secondary growth are initiated.
• All tissues inside the endodermis — pericycle, vascular bundles and pith — together form the stele.

• Treating every layer named in a root diagram as part of the bundle; only xylem, phloem and the cambium between them qualify.
• Assuming cambium is always present — monocot bundles are closed and have none.
• Confusing endodermis with epidermis; one is the innermost cortex layer, the other the outermost covering of the organ.

Usually as an odd-one-out on bundle components, or through the open-versus-closed distinction; the transport roles of xylem and phloem are the commonest follow-up.

• practice — not a real PYQ

  The vascular bundles of a monocot stem are described as closed because they

  • (a)have no phloem
  • (b)have no cambium between the xylem and the phloem
  • (c)are surrounded by an endodermis
  • (d)lie in a ring rather than being scattered

  Answer(b) have no cambium between the xylem and the phloem — without a cambium the bundle cannot form secondary xylem and phloem, so it is called closed.

• practice — not a real PYQ

  In a young root, all the tissues lying inside the endodermis together constitute the

  • (a)cortex
  • (b)stele
  • (c)epidermis
  • (d)mesophyll

  Answer(b) stele — the pericycle, the vascular bundles and the pith inside the endodermis are together called the stele.