Endosperm. Though the endosperm is usually present at some stage, it is not found in all seeds when they are mature, since it may be entirely absorbed by the growing embryo, its function of food storage being assumed by the cotyledons. It is, however, very important in many seeds, especially the grains. From its store of starch we derive our bread. Food for the embryo may be stored either in the endosperm or cotyledons. Our laboratory tests show that this stored food consists largely of starch, together with considerable proteid, a little fat or oil, and some mineral matter.
The seed has within itself the miniature plant, or embryo, and all the kinds of nutrients needed for growth except water. This the seed must get from the soil before it can grow. The growth of a seed is a very wonderful process. Though inactive, dry, and apparently dead the protoplasm is really alive and only awaits favorable conditions for growth to begin.
The insoluble, stored foods must be digested by the embryo, made soluble, united with the water which has been absorbed from the soil, and assimilated, to form all the new kinds of tissue in the growing seedling. It may seem strange to speak of a seed as digesting food, but there is a substance (diastase) in the seed, which digests its food just as truly as the fluids of our stomach digest ours. Here, then, are digestion, absorption, and assimilation going on in the seed as it begins to grow. If the food stuffs in the seed were not stored in a dry and insoluble form, they would dissolve and decay. It is necessary, therefore, if a seed is to keep over winter, that its food must be both dry and insoluble.
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