Parts of trees: The stem and roots

THE STEM

The stem is defined as the principal axis of plant from which buds and shoots are developed .In trees, stem, bole and trunk are synonymous but bole is usually referred to only the lower part of the stem up to the point where the main branches come off.  Thus, ‘clear or clean bole' is said to be that part of the bole that is free of branches.

The length and shape of the stem also varies with species and the conditions in which the tree grows. There are some species that have long and straight stem with relatively few branches, while there are others having which are crooked and/or much branched stem. Usually, the stem is thicker at the base and thinner in the upper portion of the tree. The decrease in diameter of the stem of a tree or that of a log from the base upwards is called as taper. This is due to the pressure of wind which is centered in the lower one third of the crown and is conveyed to the lower parts of the stem, increasing with increasing length. To restrain this pressure, which could snap the tree at the base, trees strengthen itself towards the base.

 

The situation in which a tree grows clearly affects the shape and length of the stem. The trees growing in the open in plains and on ridges in hills have generally shorter and conspicuously tapering stem as a result of wind pressure. Instead, the trees growing in dense forest have comparatively longer and more or less cylindrical stem. The presence of a  long cylindrical bole in a tree is a desirable quality as that increases their timber volume.

In the earlier stages of growth, thin branches usually cover almost the entire stem of a growing tree. However, as the saplings grow into poles and trees, the lower branches fall off giving in a clean bole. But sometimes in later life, may be due to some adverse factors the clean bole again develops small branches known as epicormic branches which are defined as branches originating in clusters from dormant or adventitious buds on the trunk of a tree or on older branch when exposed to adverse influence such as excessive light, fire or suppression. They are generally found on stag-headed trees as they are also caused by drought.

 

 

 

Generally, trees have one stem but sometimes they are forked and then we  have more than one leader. From the point of view of timber production, this is not a desirable quality because the portion below the point of forking is either wasted or produces small-sized timber. Also, there is always a danger of one of the leaders being broken down in wind storms.

 

In some species, e.g., Acrocarpus fraxinifolius, Bombax ceiba, Pterocarpus dalbergioides, Terminalia myriocarpa, etc., buttresses are formed in the basal portion of the stem. Buttress is out-growths formed usually vertically above the lateral roots, thus connect the base of the stem with roots. Buttresses are generally associated with the absence of long taproot which could be due to either shallow soil resulting from the presence of rock a little below the surface or badly aerated and infertile subsoil. Buttress formation, sometimes, extends upto 5m and therefore the lower portion of the stem becomes useless, unless the buttresses are very small. The felling of trees which has buttress   pose great difficulties in felling as it  has to be done above buttress formation.

In species such as Tectona grandis, the lower portion of the stem is characterized by fluting. Fluting are called as irregular involutions and swellings on the bole just above the basal swell. Fluting is considered as a defect as it decreases the basal volume considerably. It is attributed to epicormic branches, insect attack, unsuitable site or faulty thinning; however, no definite reason is so far known.

 

THE ROOT

That portion of the plant which develops inside the soil and away from light is called root. Unlike stem, roots do not produce leaves, flowers or fruits. The roots of trees support them firmly to the ground, absorb soil moisture containing mineral salts and send it to stern for onward transmission to the leaves. Generally there are two kinds of roots: the taproot and the lateral roots.

The tap root is the primary descending root formed by direct prolongation of the radicle of the embryo. In trees, tap root is the main axis of the large root system and descends vertically below the stem. Tap root is conical in shape and develops towards the permanent moisture in the soil and quite often, attains considerable length.

The roots that arise from the taproot and spread laterally to support the tree are called Lateral Roots. As the taproot grows, it develops lateral roots that are branched and re-branched and ultimately become into rootlets. The ends of the root lets are covered with fine hairs are called the root hairs. These root hairs spread in the soil particles, and absorb soil moisture and translocate it to stem and leaves where the food is manufactured. The taproot and the lateral roots including their branches up to root hairs, form the root system of the tree. The lateral roots are generally confined to the area covered by the crowns of trees but sometimes they go far beyond. For example, the lateral roots of trees growing to the edge of a forest go far into the cultivated fields and adversely affect the agricultural crops. Also in the forest, they sometimes go way beyond the area of the crowns of trees and may form root grafts with the roots of other trees in dense forest.

The roots of the seedlings develop very fast and some-times reach one meter depth in one season In favorable localities. As the roots develop much faster than the shoot in early stages, it is not possible to estimate the length of the root from that of the shoot. At Early development of taproot to a depth where the moisture in the soil is more or less permanent protects the plants is protects itself from post-monsoon and summer drought. Generally, the root requires a well-aerated soil for their growth and development. Therefore, the roots of many species, e.g., Sal, are killed by rise in the water table, though those of some species may adjust themselves according to the changed conditions. In the same manner, when a layer of silt is deposited on top of the soil, some trees may die but some others develop roots from the  portion of the stem covered and are not killed.

On the basis of the depth of the root-system, the trees are classified into shallow-rooted and deep-rooted trees. Shallow-rooted trees are that tress whose root system does not extend far enough into the soil so as to save them from relatively easy wind-throw. On contrary, the deep-rooted trees are those whose roots go very deep in the soil. The trees that develop a long taproot and large lateral roots are not easily uprooted by wind, thus, are called wind firm.

ADVENTITIOUS ROOTS

Those roots that are produced from parts of the plants other than the radicle or its subdivision are called adventitious roots. In bamboos, the roots are produced from the underground stem called rhizome and, are thus are adventitious. These roots are thin and usually undivided. These rhizomes do not show secondary thickenings and are replaced by new roots when the older ones perish. Listed below are kinds of adventitious roots are generally found in trees:

(1) Prop-roots—Ficus bengalensis is one of the best examples of adventitious-roots that it produces from its branches. They remain suspended in the air until they reach the ground. On reaching the ground, the prop roots enter it and get fixed in the soil. Because they support thick branches of the tree, they are called prop-roots.

(2) Stilt roots--Stilt roots are those adventitious roots which emerge from the butt of a tree above ground level, such that the tree appears as if supported on flying buttresses. For example, Mangroves of the genus Rhizophora.

(3) Pneumatophore—Pneumatophore are a spike-like or knee-shaped  projection of the roots of swamp tree such as Heretiera, Bruguiera, enabling the submerged roots to obtain oxygen.'

LIGNOTUBERS

In most species of Eucalyptus underground swellings called Lignotubers are found. Lignotubers are actually modified stems that are developed from double accessory buds in the axils of cotyledons. They aid in as they bear numerous buds, which become active and produce shoot, in conditions when tree is injured, cut down or burnt. They also serve the purpose of food storage. 

ROOT NODULES

Roots of a large number of plants and trees posses small nodules. These root nodules contain bacteria in them (Rhizobium) in large numbers. The bacteria which are present in the soil, enters the root in the form of bacterial filament through the root hairs. On entering the root, the filament branches rapidly and reaches inner cortex. Here it causes active cell division resulting in the formation of nodules. These nodules vary greatly in shape and size. The bacteria living in the root nodule (and not those in the soil) help in fixation of free nitrogen from the air in the form of nitrates. The plants utilize the nitrates and in exchange provide the bacteria with carbohydrates. Thus a mutually beneficial relationship, called symbiosis', is established.

There about 65 species of about 8 families where root nodules are found. Root nodules are most common in Leguminosae in which  they are found in Dalbergia, Bauhinia, Acacia, Albizzia, Erythrina, Lephrosia, Crotolaria, Indigofera and Leucaena. However, they are not found in Cassia tora. Apart from Besides Rhizobium there are several other nitrogen-fixing and root-nodule-forming bacteria.