The following points highlight the two main exercises to study various types of branching in stems.
Exercise 1:
To study various types of branching in stems.
Requirements:
Plants or models showing various types of branching.
Branching in Stems:
There are two main types of branching in stems:
(i) lateral branching, and
(ii) dichotomous branching.
(A) Lateral Branching:
In this type, the branches develop on the main axis laterally from the lateral buds.
It is of following two types:
(a) Cymose branching, and
(b) Racemose branching.
(a) Cymose or Definite Branching:
In this type of branching, the main axis terminates into a flower or a tendril, and before it terminates it gives off one or more lateral branches. It is also called definite branching because the growth of axis remains limited in this type of branching.
Cymose branching may be of following types:
(1) Uniparous or Monochasial Cyme:
Only one lateral branch is produced at each point in this type of cymose branching.
It may further be subdivided into following two types:
(i) Scorpioid cyme:
Successive branches develop on alternate sides resulting into a zigzag structure, as in grapes (Vitis vinifera, Fig. 155A).
(ii) Helicoid cyme:
Successive branches develop only on one side resulting into a helix-like structure, as in Saraca indica (Fig. 155B).
(2) Biparous or Dichasial Cyme:
Two lateral branches are produced at each point in this type of branching, as in Stellaria (Fig. 155C).
(b) Racemose or Indefinite Branching:
In this type of branching, the main axis grows for indefinite period and during this process it regularly-produces lateral branches in an acropetal order (Fig. 155D). Single axis with indefinite growth supports all lateral branches in this type, and due to this, it is also called monopodial branching (mono, single; podos, foot). Most of the angiosperms (e.g., Casuarina) show monopodial type of branching.
(B) Dichotomous Branching:
The type of branching in which the terminal bud of the axis divides into two, each forming of a branch, and in the same way the tips of these daughter branches also divide and redivide, is called dichotomous branching. In this way, in dichotomous branching, each branch bears two daughter branches, e.g., Riccia, Marchantia, Pandanus, etc.
Dichotomous branching is of following two types:
(a) Normal dichotomy, and
(b) Sympodial dichotomy.
(a) In normal or true dichotomy, both the daughter branches, formed due to dichotomous branching, are of equal dimensions, e.g., Lycopodium (Fig. 156A).
(b) In sympodial dichotomy, both the daughter branches, formed due to dichotomous branching, are of unequal growth. In such branching, one of the two branches grows more vigorously while the other branch is suppressed. Sympodial branching is also of two types, namely scorpioid dichotomy and helicoid dichotomy.
(i) In scorpioid dichotomy (Fig. 156B), suppression of branch takes place on alternate sides of successive branching.
(ii) In helicoid dichotomy (Fig. 156C), the branch of the same side is suppressed each time.
Exercise 2:
To study monopodial, sympodial and dichotomous type of branching.
Requirements:
Plants or charts showing various types of branching.
Observations and Results:
(a) Monopodial:
It is a kind of growth in which the main axis of the plant is formed by continuous growth of the same shoot apex, with lateral branches arising from it. In this type, the growth of the main axis causes height increase (Fig. 157 A).
(b) Sympodial:
It is a kind of growth in which the main axis of the plant is formed by the growth of lateral buds near the apex of the shoot, instead of by continuous growth from the apex. In this type, the growth of lateral branches causes height increase (Fig. 157B).
(c) Dichotomous:
In this type, the branches divide equally into two, and growth of all branches causes height increase (Fig. 157C).