Starch
The greater part of the food material stored up by green plants for future use in their nourishment, and in the formation of new tissues, is formed of Starch. In those parts of plants in which food is stored, e.g., in the tubers of Potatoes, in the pith of many plants, and in the albumen (or perisperm) in seeds of Cereals, Peas, and many other plants, Starch is very abundant, packed away in the cells in the form of small grains. These grains are frequently rounded, but may assume peculiar forms characteristic of certain plants. When very numerous, they become polygonal by mutual pressure. Under the microscope, each granule is seen to have a dark point, called the hilum, at one place; and around this are light and dark layers alternately. This appearance of layers, it is believed, is due to their containing a varying amount of water, as it is not apparent when the grains are either saturated or thoroughly dried. Often two or more granules are more or less completely united, forming "compound" granules. Though always small, they vary greatly in size in different plants. They are most readily recognised by running a solution of Iodine and Potassic Iodide under the cover glass, when the granules become some shade of indigo-blue, varying with the strength of the solution. Starch consists of Carbon, Hydrogen, and Oxygen, in the proportions indicated by its chemical formula, C6H10O5. This composition is the same as that of cellulose, the substance of which cell-walls are composed; and it is very nearly the same as that of several of the sugars, into which Starch is converted by a kind of slight fermentation. Starch is produced in the presence of light in the chlorophyl-bodies, and, therefore, only in the green parts of plants. There is much doubt as to the exact part played by chlorophyl; most botanists believe that it forms the Starch granules, but others follow Pringsheim in thinking that the chlorophyl is only a shield against exposure to too intense light of the protoplasm, which they regard as the actual agent in the production of Starch. Whichever view is correct, the Starch is largely built up from the Carbon and Oxygen of Carbonic Acid gas in the atmosphere. The granules are not soluble in water, so, when there is need to convey Starch from one part of the plant to another, it is changed into a sugar, which is soluble, and, in this form, passes along to where it is to be used in building up new cell-walls, or where it must be stored up for a time. In the latter case, the Starch granules are again formed by the action of small pieces of protoplasm, named Starch-generators, which are fixed to the part of each granule farthest from the hilum.