Question

Choose the correct answers from the alternatives given.
When a leaf trace extends from a vascular bundle in a dicot stem, what would be the arrangement of vascular tissues in the veins of the leaf.
A. Xylem would be on top and the phloem on the bottom.
B. Phloem would be on top and the xylem of the bottom.
C. Xylem would encircle the phloem.
D. Phloem would encircle the xylem.

Answer 1

In the dicot stem, the vascular bundles are arranged in a ring, with pith concentrated at the core of the stem. In each vascular bundle, the xylem and phloem are separated by a substance called vascular cambium.

Complete Answer:
Let us analyse the options to find the correct answer.
Option A: The dicots leaves possess reticulate venation in which veins form a network. The xylem forms the top layer and phloem is present at the bottom in the arrangement of the vascular tissues. The veins vary in thickness in the reticulate venation. Therefore, this is the correct option.

The dicot stem has following parts:
- Epidermis which is made up of multiple layers, consists of three sub-zones. It is followed by endodermis.
- The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath. It is a single layer of barrel shaped cells which have conspicuous Casparian strips.
- Pericycle present on the inner side of the endodermis and above the phloem in the form of semilunar patches of sclerenchyma.

A large number of vascular bundles are arranged in a ring; the ‘ring’ arrangement of vascular bundles is a characteristic of dicot stem. Vascular bundles are scattered in the spongy parenchyma. These can be seen in the veins and the midrib of the leaves.

Option B, C and D: As Xylem would be on top and the phloem on the bottom, therefore, these options are irrelevant. Thus, these are the incorrect options.

Therefore, the correct option is A) Xylem would be on top and the phloem on the bottom.

Note: Endarch and exarch are two types of primary xylem development in vascular land plants. Endarch arrangement results in the centrifugal pattern of development while the exarch arrangement results in the centripetal pattern of development.