Sugars in fresh plants exist almost entirely in solution. This is true of all the great sources of the sugar of commerce, viz., the palm, the maple, the sugar beet and sugar cane. This statement is also true of fruits and the natural nectar of flowers. By natural or artificial drying the sugar may be reduced to the solid or semisolid state as in the cases of raisins and honey. In certain seeds, deficient in water, sugars may possibly exist in a solid state naturally, as may be the case with sucrose in the peanut and raffinose in cotton seed.
Starches on the other hand when soluble, are probably not true starches, but they partake more or less of a dextrinoid nature. Fine starch particles occur abundantly in the juices of some plants, as for instance sorghum, where they are associated with sugar and can be obtained from the expressed juice by subsidence. But even in such a case it is not certain that the starch enters into the general circulation. It is more likely formed locally by biochemical condensation of its constituents. Starches in a soluble or semisoluble state are transported, as a rule, to the tubers or seeds of plants where they are accumulated in large quantities as a reserve food for future growth. For a study of the plant metabolism whereby starch is produced and for its histological and physiological properties the reader may consult the standard authorities on vegetable physiology.[170]
204. Sugar in the Sap of Trees.—Many trees at certain seasons of the year, carry large quantities of sugar in their sap. Among these the maple and sugar palm are preeminent. The sap is secured by cutting a pocket into the side of the tree or by boring into it and allowing the sap to run into an appropriate receptacle through a spile. The content of sugar in the sap of the maple and palm varies greatly. In some cases it falls as low as one and a half and in others rises to as much as six or seven per cent.[171] In most cases the sugar in the maple sap is pure sucrose, but towards the end of the flowing season it may undergo changes of a viscous nature due to fermentation, or inversion, forming traces of invert sugar. In this country the sap of the maple may flow freely on any warm day in winter, but the sugar season proper begins about February 15th in Southern Ohio and Indiana, and about March 25th in Vermont. It lasts from six weeks to two months. The sap flows best during moderately warm, still days, after a light freeze.
In addition to sugar the maple sap contains a trace of albuminoid matters and some malic acid combined with lime. As a rule it can be subjected to polarization without preliminary clarification.
205. Determination of Sugar in Saps.—In most cases the sap may be directly polarized in a 200 millimeter tube. Its specific gravity is obtained by a spindle or pyknometer, and the percentage of sugars taken directly from the table on [page 73], the degree brix corresponding to the sugar percentage.
On polarizing, the sugar percentage is calculated as follows:
Multiply the specific gravity of the sap by 100 and divide the product by 26.048. Divide the direct reading of the sap on the sugar scale by the quotient obtained above, and the quotient thus obtained will be the correct percentage of sugar in the original solution.
The formula is applicable for those instruments in which 26.048 grams represent the normal quantity of sugar which in 100 cubic centimeters reads 100 divisions on the scale. When other factors are used they should be substituted for 26.048 in the above formula.
The principle of the calculation is based on the weight of the sap which is contained in 100 cubic centimeters, and this is evidently obtained by multiplying 100 by the specific gravity of the sap. Since 26.048 is the normal quantity of sugar in that volume of the solution the quotient of the actual weight divided by that factor shows how many times too great the observed polarization is. The simple division of the polariscope reading by this factor gives the correct reading.
Example: Let the specific gravity of the sap be 1.015 and the observed polarization be 15.0. Then the true percentage of sugar in the sap is found by the equation: