In this article, we propose to discuss the two types of branching of stems.
They are as follows: (1) Lateral or Axillary Branching and (2) Dichotomous Branching.
A. Lateral or Axillary Branching:
Branching from lateral buds is the rule among flowering plants. As the lateral buds are usually axillary, this may also be called axillary branching. Branching is racemose or cymose according as the lateral buds are less vigorous or more vigorous than the apical bud.
(A) Racemose or Monopodial Branching:
Very often the apical bud of the plant grows indefinitely giving rise to a straight stem axis or podium on which the lateral buds grow in an aeropetal order. Since there is a single axis for the shoot, the type of branching is monopodial.
Other names for this type are racemose and indefinite—the last name because in this type the possibility of vertical growth of the plant is unlimited. This type of branching is the commonest among young Phanerogamic plants.
But, as the plant grows older, the apical bud often gets lost and some side branches become very strong so that the plant loses its monopodial character. Typical monopodial branching can be seen in the excurrent trees like the conifers or the mast tree (Polyalthia longifolia), Casuarina, etc.
If, instead of there being a single leaf at each node there be a whorl of leaves, the branches at each node will also be whorled as in Alstonia scholaris .
(B) Cymose Branching:
In many tropical trees, as the plant develops, the apical bud gets lost and die side branches develop forming a spreading tree. This gives a deliquescent form found special among trees growing in the open, e.g., mango or jack-fruit.
In forests, however, the lower side branches cannot develop because of the absence of light. The plants here are always tall like Shorea robusta.
As opposed to the above, in some plants it is the rule that the apical bud does not develop from the early stage and the branching of the plant is dependent solely on the lateral branches immediately below the apex. This is called cymose branching.
This may also be called the definite type of branching as the vertical growth of the plants is limited. These plants cannot be very tall.
There may be the following types of cymose branching according as one, two, or more strong branches develop below the apex:
(i) Uniparous Cymose:
Only one strong branch develops below the stern or the branch apex at each branching. The leaves are arranged spirally so that there is only one axil just below the apex. Among uniparous types again two patterns are distinct:
(i) In some, the successive branches below branch tips are alternately to the right and the left. This will give a zigzag form to the stem . In nature, however the plant does not remain zigzag but straightens up – forming a false axis or pseudopodium.
The pseudopodium is also called sympodium as the axis is formed by joining up successive branches. This is a scorpioid cyme. This is very prominently seen in the vines of different species of Vitis . In these, the apical buds arc transformed into tendrils which are pushed to a side by the straightening up of the sym- podium. That is why the leaves are apparently placed opposite the tendrils.
(ii) In the other type of uniparous cyme the branches are successively on one side. This should give rise to a spiral appearance , hence the name helicoid cyme. But, as in the other case, the pseudopodium straightens up and the real nature is understood only by observing that all the leaves are on one side while all the branches lie opposite.
This is found in Saraca asoca. The scorpioid cyme again may be rhipidium or cincinnus and the helicoid may be drepanium or bostryx according as all the lateral branches are in the same plane or not.
(ii) Biparous Cymose:
Here, instead of a single lateral shoot developing at each branching, two such branches develop.
This usually happens when there are two opposite leaves at each node and, therefore, there are two opposite axillary buds.
This gives rise to a symmetrical shoot known as dichasium. Very often the apical buds die off early so that no trace of them remains and then it seems that the apical bud has bifurcated giving rise to two branches.
As this is like true dichotomy (described below) it is sometimes called false dichotomy. This is seen in Mirabilis jalapa, Tabernoemontana divaricata, Plumeria acuminata, Carissa carandas, Mistletoe etc.
(iii) Multiparous Cyme:
If there be more than two leaves at each node and if the apical bud does not develop, there will be more than two branches at every step. In Nerium odorum (oleander) leaves are borne in whorls of three and the branching is of the trichasium type. Polychasium may be seen in Croton bonplandianum.
B. Dichotomous Branching:
Although this type of branching is practically confined to the Cryptogams it is necessary to have some idea of it. The growth of such a plant is by a single apical cell or by a group of such cells which bifurcates or splits giving rise to two apical branches.
The tips of these branches again bifurcate and this splitting goes on in branches of higher orders. This is normal or true dichotomy . In other cases, after the bifurcation of the tip, one half grows normally while the other half is practically suppressed.
This is called sympodial dichotomy as this type of branching gives rise to a false axis formed of the bases of successive branches (sympodium) just as in the case of uniparous cymose branching. If the branches on one side only are suppressed the plant should take up a coiled structure and the branching is called helicoid dichotomy.
On the other hand, if alternate branches on the left and the right are suppressed, the zigzag appearance causes the name scorpioid dichotomy. In nature, however, these curved stems straighten up forming false axes or sympodia.
The types of dichotomous branching should be compared with similar cymose forms.