Under the bark of a tree lies a bundle of hard-walled tubes part of the circulatory system of the tree. The larger tubes, or ducts, carry water with its minerals from the root hairs up the stem and into the leaves. Other ducts carry the manufactured food in the leaves down to all parts of the plant.
The roots of plants contain root hairs; each root hair is along, hair-like cell with a membrane around it. Soil water and minerals enter plants through these membranes by diffusion and circulate throughout the plant.
When the food has been manufactured in the leaf, another System of the tubes distributes it. The tubes which carry water to the leaves are called xylem; the food-carrying tubes are called phloem.
Food circulation in plants
Since plants do not possess a heart to pump liquids through it, it must depend on the process of diffusion to speed the materials it needs through the protoplasm of the cells. The circulatory systems of some plants rest in the autumn when their leaves drop, off and the sap containing the manufactured food is stored in the lower part of the trunk. In the spring, the liquid food rises in the tree and with the development of leaves, photosynthesis begins.
Digestion of food
Digestion is the process whereby foods, which are insoluble or very complex chemically, are converted into foods which dissolve more readily in water or which are simpler chemically. Only water-soluble foods can be moved from one part of plant to other parts. Digestion proceeds virtually in all living cells, but is especially active in the cells of tissues which store considerable quantities of food.
Liquids and solids dissolved in liquids are able to pass from cell to cell by osmosis. By osmosis the sugar may travel from cell to cell in the leaf until it comes to the veins, along which it travels to the stern and so throughout the plant. Food material is usually found in plants in the form of starch and other compounds which cannot be solved.
The question arises, how it is that food can be transferred to the various parts of the plants where it is needed. Certainly the solid grains such as grains of starch cannot travel far through the plant unchanged.
Digestion of starch
The starch (the pure fecula or white farinaceous matter of plants) does not ordinarily dissolve in water, but it is rather easily changed into sugar, and the sugar will dissolve. If you examine with a microscope some starch grains from a sprouting seed, where the food material is being carried away to be used in forming new plants, you will find that the grains of starch are not smoothly rounded. As they would be at any other time, but rough with many holes on the surface, as if some of the starch had been dissolved from each grain. Plants have a substance (called diastase) which has the ‘peculiar property of causing starch to change slowly into sugar if the diastase is brought in contact with the starch. This substance is in the plant sap (the vital juice of plants).
It acts upon the staph at the surface of the grains, and the rough, pitted appearance shows where the starch has been. So changed and dissolved. This process of changing an insoluble substance into a soluble one is caused by digestion. Insoluble food can be transferred from place to place through the plants only after they have been made soluble by digestion.
The dissolved food materials may pass from cell to cell throughout the plant. When food reaches the large pores or tubes in the stem, it may pass upward with the general movement of water. Passage in a downward direction occurs through tubes in the inner bark. Immediately within the tissues through which the manufactured food passes down and, is the layer of tubes through which water passes upward in the stem. The veins of the leaves are composed of tissues similar to those of living wood and bark, and liquid material moves in them as it does in the vood and bark.
When the food is in solution and is capable of being transferred from place to place within the plant, it may either be used immediately for repair and growth or maybe stored.
When food is stored it is usually changed into an insoluble form again. As for example, sugar is commonly changed into starch. Food may be stored in almost any part of a plant, but usually, it is stored in large quantities only in the-thickened part—examples of thickened various bulbs and tubes, ­ fruits and seeds: Very often food is deposited in the roots and other underground parts of the plant. In these cases the food which is made and stored during the summer commonly serves for the plant’s growth during the earlier part of the following seasons.
The large amount of food which is brought together within a small space makes these storage organs of plants an important article of food for men and animals. Wheat, rice, beans, peas, corn, oats are all stored foods.