The stems
Metamorphosed Aerial stems: Aerial
stems undergo extreme modifications i.e.metamorphosis like all other organs to discharge various functions other than normal. These are as follows:
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| Twigs of vitis showing apical tendrils |
(a) Stem or Branch tendril :--It is a metamorphosed stem or branch, slender and coiled, meant for the support of weak stems. In many cases, as in vitis the upper extremity is transformed into a coily slender tendrillar part. From the axil of leaf on the lower unchanged part another branch comes out whose apical portion is similarly transformed into a tendril and so on; the branches are thus, arranged in scorpioid cyme. The pseudo axes or branches become straight with the leaves opposite to the tendril. In Passiflora foetida (Passifloraceae), the tendril is originated in the axil of the leaf. Function :Serves as a climbing organ.
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| Thorns of different specimens |
(b) Thorn :-It is an axillary branch. Its further growth is arrested or checked and ultimately converted into a hard, sharp and pointed structure. Sometimes it bears small leaves, otherwise its axillary position indicates its branch nature. Examples: Duranta repens (Verbenaceae), Alangium salvifolium (Umbelliferae) etc.
Function-The thorn serves as a defensive organ.
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| Different phylloclades |
(c) Phylloclades or Cladophylls and Cladodes-Phylloclade or cladophyll is a flat leaf-like aerial metamorphosed stem or branch of many or several nodes and internodes doing the function of leaves. In many plants growing in dry or and regions, leaves are reduced to spines or leaves fall off very early as an adaptation and precaution against loss of water. The phylloclades function as leaves and sometimes act as water storing organs. Examples-Opuntia dillenii (Cactaceae), Muehlenbeckia platyclados (Polygonaceae') etc. are typical phylloclades with distinct nodes and lnternodes.
In some plants like Asparagus racemosus (Liliaceae) and Ruscus aculeatus (Liliaceae), each branch consists of one internode arising at the axil of minute scale leaves. The branches become very much leaf like. These metamorphosed leaf-like branches of one internode are called cladodes. Phylloclades and cladodes do the function of foliage leaves i.e. photosynthetic.
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| Pseudo-bulbs |
(d) Pseudo-bulb :---It is also a modifled stem as seen in aerial orchids. Here generally one internode of the stem becomes modified into a fleshy and tuberous structure. Its function
is to store moisture in excess so that the plant can survive during unfavourable condition.
(e) Bulbils :-These are aerial bulbs arising at the axil of foliage leaves or scale leaves. Sometimes they consist of disc-like stem covered by leaves as in Agave sp. Bulbils are also present in Lilium bulbiferum (Liliaceae), Globba bulbifera (Zingiberaceae). In Agave cantala (Agavaceae) the bulbils are found to produce new plants while still attached to the stem-hence the bulbils are viviparous.
Branching :
Definition-A branch is a similar appendage of the axis having nodes and internodes. It develops at the axil of a leaf and bears leaves like its mother axis.
All branches or stems have endogenous origin. In angiosperm stems, branches arise from buds developing in the axil of leaves and have the vascular supply from the stele of
the main stem. The manner in which the branches are arranged on the stem is known as branching.
Types of branching-There are two principal types of branching viz., 1. Lateral and
2. Dichotomous.
I. LATERAL BRANCHING :-Here the branches are given off laterally from the lateral buds of the main axis. The branches are produced from the sides of the main stem.
Lateral branching is of two types viz. (a) cymose and (b ) racemose.
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| different types of branching |
A. Cymose or definite branching :--In this type of branching the main axis gives off one or more branches slightly below its base from the axillary buds, the main axis does not grow further in length. The growth of the apex is stopped due to its termination in a tendril
or flower bud. Hence the growth of the main axis is definite. This type of branching is known as cymose.
Cymose branching is of three kinds :
(a) Uniparous :-When only one branch is produced from the main axis at each branching, then it is called uniparous cyme. It may be scorpioid or helicoid.
(i) Scorpioid-When lateral branches are produced on alternate sides in a zig-zag manner, e.g. Cissus quadrangularis, Vitis vinifera (Vine) etc. of the family Vitaceae.
(ii) Helicoid-When lateral branches are produced on one side only e.g. Saraca indica (Caesalpinaceae of Leguminosae).
(b) Biparous :-When two branches arise just below the main axis, it is known as biparous cyme-e.g. Mirabilis jalapa (Nyctaginaceae), Carissa carandas (Apocynaceae) etc.
In case of the Plumeria acutifolia (Apocynaceae) the terminal bud remains undeveloped or soon dies off ; it looks like a false dichotomy.
(c) Multiparous-When more than two branches are produced, it is called multiparous cyme-e.g. Croton bonplandianum (Euphorbiaceae).
B. Racemose or Indefinite branching :---In most angiosperms the main axis grows indefinitely giving off axillary branches in acropetal order, the terminal bud grows for an indefinite period and persists throughout the whole life of the plant. In this type of branching the primary axis or stem supports the lateral branches ; therefore it is also known as monopodial branching. Examples-Polyalthia longifolia (Annonaceae), Lawsonia inermis (Lythraceae),
Hibiscus sp. (Malvaceae) etc.
II. DICHOTOMOUS BRANCHING :
In this type the apical mass of tissue bifurcates due to the division of a single or a group of pyramidal cells at the apex, which by further divisions produces two branches of equal dimension in a forked manner. The tips of daughter branches may again fork in the same manner, e.g. found chieHy in Dictyota dichotoma (alga), Lycopodium (pteridophyte)
and Clinogyrte dichotoma (Marantaceae-an angiosperm).
It may be of two kinds e.g. (1') normal and (ii) sympodial
(i) Normal dichotomy :-Here the tip of the main axis bifurcates forming branches of equal dimensions. The daughter axis branches in the same manner as the main axis, e.g.
Lycopodium clavatum, Psilotum tripuctrum-pterjdophytes.
(ii) Sympodial dichotomy :-Here the tip of the main axis bifurcates forming two branches of unequal dimensions. One of the daughter axis grows more vigorously than the other.
It may be either, (a) Helicoid i.e. one sided branching or (b) Scorpioid, branching on alternate sides.
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| Classification of branching |
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