It is apparent that the production of these immense stores of reserve food by plants makes them useful as food for animals, and it is, of course, the storage parts of the plants which are most useful for this purpose. This biological relationship needs no further emphasis.
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If the organic compounds produced by plants be classified with reference to their uses in metabolism into the three groups known, respectively, as temporary foods, storage products, and permanent structures, it is clear that the carbohydrates which have been discussed in this chapter may fall into either one of the first two of these classes. There can be no doubt that the first products of photosynthesis, whichever ones they may be in different plants, may be directly used as temporary foods, to furnish the energy and material for the building up of permanent structures.
Also, there can be no doubt that these same carbohydrates are translocated to the storage organs and accumulated for later use by the same plant (as, for example, in the case of the perennials), or by the next generation of the plant (when the storage is in the endosperm adjoining the embryo of the seed). There is no known explanation as to why different species of plants make use of different carbohydrates for these purposes; or why certain species elaborate starch out of the same raw materials from which other species produce sugars, inulin, or glycogen, etc.