Though all sugar-canes are members of the great family of grasses, there are remarkable variations in the size, general appear- ance and habit of growth of different varieties. Some of the wild canes are slender with fine narrow leaves and are closely similar to the taller grasses, while commercial canes used for the produc- tion of sugar have thick stalks, broader leaves and extensive root systems.
A description of the plant from the point of view of the planter may well begin with the stalk or stem which consists of a succes- sion of joints or nodes spaced at intervals and separated by what are called internodes. At each joint or node is a bud, successive ones appearing on opposite sides of the cane stalk. Growing from the lower part of the joint is the leaf sheath which completely surrounds the stalk for a distance of several inches up to about a foot in an upward direction before it develops into the char- acteristic leaf blade. The sheath is in the form of a cylinder severed vertically down one side, the bud being contained within the sheath on the opposite side to the vertical opening. T h e sheath is therefore attached to the stalk by a basal ring clearly visible as a slightly thickened part of the joint when the leaf sheath is removed. Immediately above this are rings characterized by a number of light-coloured spots which are embryo roots, called the root or growth band (Fig. 7).
The distance between the joints, i.e., the length of the internode, varies considerably even with the same variety of cane under different conditions. If the cane is subject to drought or continu- ing cool weather, growth is arrested and the distance between the joints decreases. When conditions favourable to growth return, this distance, that is to say, the length of the new internodes, increases. It is, however, one of the characteristics of the different
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varieties under normal conditions of growth. Some canes display a staggered arrangement of successive internodes each being set at an angle to the other giving a zigzag appearance to the stalk.
As a general rule types of cane which have comparatively long internodes and straight stalks are preferred.
The stalk is more or less cylindrical, with an outer rind varying in hardness with the variety of cane, and consists internally of softer tissue, comprising fibrous ducts and their walled cells. T h e fibres are bundles of extremely fine tubes some conveying plant foods in solution from the roots to the leaves, and others carrying the juice, with the dissolved sugar and other products synthesized in the leaves to the cells in the stem. T h e nodal portions are much tougher than the internodes. The surface of the stem is coated with a thin protective layer of waxy material which varies in amount with the type of cane, and is moisture resistant.
Varieties differ considerably in the extent to which the leaf sheaths adhere to the stalk as growth proceeds. In some, as the older leaves die and dry up the sheath becomes loose on the stalk and breaks away easily at the point of attachment. Canes of this type are said to be free trashing. Others retain the older leaf sheaths more firmly, with the blades attached. T h i s feature is undesirable in very wet conditions because water collects at the base of the leaf sheath, and if wet weather persists the buds may grow and roots develop. Where the moth borer is a serious pest the persistent leaf sheath protects the eggs and the caterpillars which develop from them against access by natural enemies, whether these are bred for distribution in the cane fields, or norm- ally present as part of the biological population. At the same time certain varieties which are retentive of their trash in this way are vigorous, heavy yielding, rich canes, advantages which outweigh the drawbacks caused by the absence of free trashing.
T h e R o o t s
The root system of the cane consists of comparatively thick buttress roots which may be seen at the base of the stalk. These help to support the cane but play little or no part in supplying nutrients. The root system proper consists of a mass of small roots confined mainly to the upper 12-in. layer or so of the soil, with some which penetrate to depths of 10 to 12 ft. or more under
some conditions. This principal root system of the cane provides the means by which the plant obtains through the root hairs supplies of food for growth and development. A healthy, vigorous and strong root system is a desirable characteristic of cane for the support of the plant as well as for the ample supply of food requirements. Varieties of cane show differences in the vigour and extent of their root systems, and as mentioned elsewhere some shallow rooting varieties are easily blown over in wind storms, while the vigour of others enables attacks by root damaging pests to be defeated by the rapid development of new roots.
Flowering
The stalk terminates at the upper end in a whorl of developing leaves and a growing point, which under certain conditions develops into a slender arrow bearing a tassel of tiny flowers very similar to an enlarged edition of the seeding heads of some grasses (Fig. 10). The ability of cane to flower and to produce fertile seed is important for cane breeding work, but is an undesir- able characteristic in commercial canes, since the appearance of the arrow and tassel indicate that the cane has completed its vegetative growth and is rapidly becoming mature. The conditions under which canes may be induced to flower, or by which flower- ing may be suppressed are being studied. In Hawaii for example it has been demonstrated that exposure of the cane to strong electric light for short periods during the night, at the period of growth when arrowing normally occurs, prevents formation of the flowering head and enables cane to continue growing, or in other words causes the plant to continue "making cane."
Function of the Leaves
The complex processes whereby sugar is built up or synthesized from the simple chemical compounds water and carbon dioxide, which are respectively taken up by the plant through the roots and the leaves, take place in the leaf under the influence of sun- light. The green colouring matter of the leaf, known as chloro- phyll, plays a highly important part in this transformation.
Agencies which damage the younger leaves, restrict their develop- ment, or destroy the chlorophyll can cause a stunting of the whole plant and a reduction in the amount of sugar produced.