. Biology and man. Biology; Human beings. hemoglobin content. Rats fed on milk from the cows in such regions show nutritional deficiencies and die in large proportions unless minerals are added to their diet. It is possible that some of the elements or compounds which we find in various plant and animal bodies are residues of material taken in but no longer used by the protoplasm; that is, they are waste products. In some species, for example, the roots or the underground stems contain crystals of a calcium compound, calcium oxalate, which we can recognize by the acrid taste, as in jack-in-the
. Biology and man. Biology; Human beings. hemoglobin content. Rats fed on milk from the cows in such regions show nutritional deficiencies and die in large proportions unless minerals are added to their diet. It is possible that some of the elements or compounds which we find in various plant and animal bodies are residues of material taken in but no longer used by the protoplasm; that is, they are waste products. In some species, for example, the roots or the underground stems contain crystals of a calcium compound, calcium oxalate, which we can recognize by the acrid taste, as in jack-in-the-pulpit. These crystals may represent wastes resulting from metabolism or leftovers from processes in which the plant has more calcium than the living protoplasm can use (see illustration, p. 215). From actual experience and special experiments, we know that some of the elements found in protoplasm are indispensable—sodium, potassium, calcium, phosphorus, magnesium, iron and chlorine, for example. Where farming goes on year after year, some of the minerals from the soil are carried off with each crop. In time the soil can no longer maintain the plants. In this country, farming has in the past consisted largely of work- ing fields until they could yield no more, and then moving on. For this reason many of the abandoned farms are quite worthless. Special Elements Some dozen elements take part in the growth and activity of most kinds of protoplasm. In addition, many species use certain minerals in special ways. The bones and teeth of vertebrates, for example, are typically hard and rigid. We find that they contain very large propor- tions of calcium phosphate, which consists of calcium, phosphorus and oxygen. Again, the shells of moUusks consist of almost pure calcium car- Larynx Parathyroid glands behind thyroid Trachea-. Thyroid gland The food and water which the organism takes in contain a very small proportion of iodine. The product of the thyroid gland, however, the thyroxi
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