=Mannan= bears the same relation to mannose that starch does to glucose and inulin to fructose. It occurs as a reserve food substance in many plants. It has been reported as present in moulds, and in ergot; in the roots of asparagus, chicory, etc.; in the leaves and wood of many trees, such as the chestnut, apple, mulberry, and many conifers; also as a part of the so-called "hemi-celluloses" which are present in the seeds of many plants, notably the palms, the elders, cedar, larch, etc. It is a white, amorphous powder, which is difficultly soluble in water, is strongly dextrorotatory (specific rotatory power +285°), and when hydrolyzed yields mannose. =Secalin= (or carubin) is a substance which is found in the seeds of barley, rye, etc., which is similar to mannan, but is optically inactive. (D) GALACTANS These bear the same relation to galactose that the preceding dextrosans do to their constituent hexoses. Four different galactans have been isolated from plant tissues; they are all white, amorphous solids which dissolve with difficulty in water, forming gummy solutions. Both galactans and mannans commonly occur associated with cellulose and hemi-celluloses in the seeds or other storage organs of plants. They are practically indigestible by animals, as the proper enzymes to hydrolyze them are not present in the digestive tract; hence, they are commonly classed with the indigestible cellulose as the "crude fiber" of plants which are to be used as food by animals. PHYSIOLOGICAL USE AND BIOLOGICAL SIGNIFICANCE OF CARBOHYDRATES 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.