Question

In 1/2 distichous phyllotaxy
a. ${2^{nd}}$ leaf lies on ${1^{st}}$ leaf at $180^\circ \;$ angle.
b. ${3^{rd}}$ leaf lies on ${1^{st}}$ leaf at $180^\circ$ angle.
c. ${1^{st}}$ leaf lies exactly below ${2^{nd}}$ leaf at $120\;^\circ$ angle.
d. None of the above.

Answer 1

Phyllotaxy refers to the arrangement of leaves on the stem of a plant. Phyllotaxy ensures a systematic arrangement and pattern of leaves. Alternate and opposite phyllotaxy are most commonly observed and a two-ranked leaf arrangement is a special case that involves either both or one of them. This is seen in bulbous plants.

Complete answer:
- As mentioned, phyllotaxy is the pattern by which the leaves are arranged on the stem. There are different types of phyllotaxy that are observed in plants.
- Distichous phyllotaxy is observed in plants belonging to the family Graminae. In this arrangement, the leaf that is present on the second node is directly opposite the leaf present on the first node. The third leaf is present above the second leaf and the fourth leaf is present above the second leaf.
- The angle of divergence between the first and third leaf is $180^\circ$ . This implies that this is a straight angle.
- So, we can understand that in this type of phyllotaxy, the leaves are present on two vertical rows and opposite sides of the axis.
- Distichous phyllotaxy can be observed in Monoon longifolium. It is a species of a small tree that can be found in many parts of Asia.

Hence, the correct answer is option (B).

Note: Mainly, there are three types of phyllotaxy:
- Alternate: in this type of phyllotaxy, a single leaf is produced at a node in an alternate pattern
- Opposite: in this type of phyllotaxy, a pair of leaves are present on a single node, opposite to each other.
- Whorled: in this type of phyllotaxy, more than two leaves arise from a single node and form a whorl.