Contents:
Tree Barks: Definition, Shapes, and method of collecting
Definition
The secondary external tissues lying outside the cambium in stem or root of dicotyledonous plants, are known as the bark.
Botanically, bark is also known as periderm. Periderm consists of three layers viz., cork (phellem); cork-cambium (phellogen) and secondary cortex (phelloderm). Commercially, barks consist of all the tissues outside the cambium. A young bark includes epidermis, cortex, pericycle and phloem. Barks are obtained from the plants by making longitudinal and transverse incisions through the outer layers followed by peeling. Barks may be obtained from stems or roots. Due to the excessive growth produced by the cambium and cork cambium, the external tissues get tangentially stretched or torn and hence, the epidermis is not found in the barks.
Characteristics of Barks:
Barks exhibit several morphological and microscopical characters. The morphological characters need special attention, as they help in the identification of the barks.
Shapes in Barks:
The shape or form of the bark is dependent upon the method adopted for its preparation. It also depends on the type of incision made and the extent of any subsequent shrinkage of the tissues. When the bark is removed from the large trees and dried under pressure, the flats are produced, e.g. quillaia and arjuna. When the bark is removed from the small branches due to shrinkage of the soft tissues, it tends to curve forming concavity on the inner side, yielding curved pieces, e.g. wild cherry and cassia. If the concavity is on the outer side of the bark, it is described as recurved, e.g. kurchi.
When the shrinkage of the tissues is to a greater extent and it forms deep trough or channel, it is called a channelled bark, e.g. ashoka, Cinchona ledgeriana and cassia. In some cases, one edge
of a bark covers the other to form quill, e.g. cascara and cinnamon. If both the edges of the bark roll independently forming quill, it is described as double quill e.g. Java cinnamon. In some cases, one quill of a bark is put inside other quill to form a compound quill, e.g. cinnamon. Compound quill is a man-made shape of bark. It reduces the exposure of bark to atmospheric conditions and also saves the space in transport.
Fractures in Bark:
The appearance shown by the transversely broken surfaces of the bark is known as fracture. It is, sometimes, useful in identification of barks. The types of the fractures are as follows.
When fractured surface is smooth, it is described as a short fracture (cinnamon and kurchi). If the exposed surface exhibits small rounded appearance, it is described as a granular fracture (wild cherry and cassia). If the broken surface shows the presence of uneven projecting points, it is described as splintery fracture, as seen in cinnamon. The presence of numerous fibres on the transversely broken surface is described as a fibrous fracture (cinchona). If the exposed surface shows the arrangements of layers one over the other, it is described as a laminated fracture, as observed in quillaia.
The various characters shown by the barks on the outer, as well as inner surface are also diagnostically important. Amongst these, the colour, condition and presence of several growths like lichens, mosses etc., are characteristic to each bark. The presence of lenticels and development of cracks are additional characters of bark. Outer surface of the bark shows presence of cracks and fissures, which are due to lack of elasticity or due to increase in girth of the trees.
Fissures are usually deep. Wrinkles, which are seen on outer surface of the bark, result due to shrinkage of inside soft tissues. Furrows are troughs between wrinkles. Inner surface of bark shows characteristics such as striations, which are longitudinal and parallel lines. Transverse wrinkles present on inner surface are described as corrugations.
Methods of Collecting Barks:
Barks are collected in a season when they contain a maximum concentration of active constituent. Cinnamon is collected in the rainy season, while wild cherry is collected in autumn.
Following are the methods of collecting barks.
1. Felling method: This is a very old method of collecting barks. The tree is cut at base and bark is peeled out. This method is not used at present commercially, since it causes total destruction of trees.
2. Uprooting method: In this case, the roots of plant are dug out of soil and bark is stripped off from roots and branches. This method is applied for collection of root bark of cinchona in Java.
3. Coppicing method: In this method, the plant is allowed to grow for a definite period and then it is cut off at specific distance from soil. The stumps, which remain in ground are allowed to send shoots, which develop further independently yielding aerial parts. These new parts are cut off and bark is collected from shoots. As compared to other methods of collection of bark, this technique is more economical and less time-consuming. It is, therefore, the method of choice for collecting barks commercially. Cascara and cinnamon are collected by this method.
Microscopic Characters of Barks:
General microscopic characteristics of barks are represented in Fig. 2. Depending upon several factors such as exfoliation of bark or special technique of preparation of bark for market as in case of cinnamon, histological structures may vary from bark to bark.