Question

Number of orthostichies present in opposite and decussate phyllotaxy is
A. Two
B. Four
C. One
D. Three

Answer 1

Phyllotaxis or phyllotaxy is a term used to describe the arrangement of leaves on a plant stem. Phyllotactic spirals form a distinctive class of patterns in nature. The term was coined by Charles Bonnet who describes the arrangement of leaves on a plant.

Complete answer:
Phyllotaxy: It is the arrangement of the leaves on the stem.
It is of three types:
- Alternate
- Opposite
- Whorled

In opposite type of phyllotaxy: In this pair of leaves is present in the opposite direction at each node or at each level. Opposite pair can be thought as a whorle of two leaves.
It is further divided into:
- Decussate phyllotaxy
- Superposed phyllotaxy.

In the decussate type of phyllotaxy: In this type two successive pairs of leaves occur perpendicular to each other. It is a common member of the family of crassulaceae. Many species have two fully developed leaves at the same time. Whorled arrangement is unusual on plants except for those having short internodes. A whorl can occur as a base structure where all leaves get attached at the base of shoots and internodes.

Orthostichy: It is the term in which a row of leaves are along the stem. All the leaves lie in a fixed number of orthostichies.
Decussate and opposite types of phyllotaxy have two orthostichies. Examples include Catharanthus roseus.
In a superposed type of phyllotaxy, the pairs of leaves are in the same plane.
For example: Combretum.

Hence, the correct answer is option (A).

Note: Distichous phyllotaxy is also called "two-ranked leaf arrangement" is a special case of either opposite or alternate leaf arrangement where the leaves on a stem are arranged in two vertical columns on opposite sides of the stem.
Examples include various bulbous plants such as Boophone. It also occurs in other plant habits such as those of Gasteria or Aloe seedlings, and also in mature plants of related species such as Kumara plicatilis.