With the development of the subject of vitamines, attempts have been made to trace the origin of these elusive substances. It is now pretty well conceded that the animal cannot synthesize vitamines; and therefore we are forced to return to the plant kingdom, from which, in the ultimate analysis, all our food is derived.

Evidence has been presented by a number of American and English investigators to show that plants, and particularly yeast (which has been much studied) possess the property of synthesizing vitamine. On the other hand, with reference to yeast, it is questionable whether, though it has the power of synthesizing vitamine, it actually needs vitamine for its growth. Dr. McCollum writes: "The results of the efforts to cultivate yeast in nutrient solutions containing no possible source of the antineuritic factor (that is, water-soluble B, a vitamine in which yeast is particularly rich) have been of such a nature as to make us seriously question whether yeast is dependent on a supply of the antineuritic principle for its continued multiplication." Perhaps, as has been pointed out in the section on vitamine B, the accelerator of yeast growth may be due to a fourth vitamine almost invariably admixed with vitamine B - the so-called vitamine D (Funk); but this subject needs further study.

A number of attempts have been made to make use of the accelerating effect that vitamine B has upon the growth of yeast in order to develop a quantitative method for the estimation of vitamine B in different substances (see page 167); these have not met with much success.

Some details of two experiments will now be given in order to acquaint the reader with the kind of evidence presented to show that vitamines are synthesized by plant tissues.

Is yeast, grown on a medium devoid of vitamine B, able to produce this substance? Such was the question that Drs. Harden and Zilva (among others), of the Lister Institute, London, set themselves to answer. They grew the yeast on a synthetic medium containing ammonium phosphate and ammonium chloride as sources of nitrogen, together with the necessary mineral salts and cane sugar (which was fractionally precipitated by alcohol from an aqueous solution, and a solution of the dried, purified material then shaken three times with Fuller's earth to remove any possible traces of vitamine B). The yeast was centrifuged out of the medium, washed three times with distilled water, pressed and dried in air. The dried yeast was then compared as regards its curative effect on pigeons suffering from polyneuritis (the equivalent of beriberi in man) as a result of a diet of polished rice. Cures were obtained, tending to show that the yeasts had produced vitamine.

In the next experiment, due to Miss Coward and Dr. Drummond, of University College, London, evidence is presented to show that vitamine A is formed in living plant tissues. In the first place, germination of seeds, such as carrot, turnip, cabbage, cress, peas and white and yellow maize, which has been shown to increase vitamine C (see pages 151-2), did not effect their content of vitamine A, which remained poor before and after germination (as determined by tests on rats). Etiolated seedlings, grown in sand, did not contain vitamine A, but green seedlings, - obtained by growing in sand, watering with nutrient solution (containing pure mineral salts only), and exposing the seeds to light - such as the green shoots of turnip, maize and peas, possessed a much higher value as a source of vitamine A than either the seeds from which they sprang, or the corresponding etiolated leaves. Water culture experiments with Tradescantia ("Wandering Jew"), using an inorganic culture medium, seemed to show that vitamine A can be synthesized by the green plant from inorganic material. (References: A. D. Emmett and M. Stockholm, Journal of Biological Chemistry, volume 43, page 287, 1920; A. D. Emmett, Journal of Industrial and Engineering Chemistry, volume 13, page 1104, 1921; R. J. Williams, Journal of Biological Chemistry, volume 38, page 465, 1919; F. M. Bach-mann, Journal of Biological Chemistry, volume 39, page 235, 1919; C. Funk and H. E. Dubin, Journal of Biological Chemistry, volume 44, page 487, 1920; F. K. Swoboda, Journal of Biological Chemistry, volume 44, page 531, 1920; M. Ide, Journal of Biological Chemistry, page 521, 1921; A. Harden and S. S. Zilva, Biochemical Journal, volume 15, page 438, 1921; M. B. MacDonald and E. V. MeCollum, Journal of Biological Chemistry, volume 45, page 307, 1921; V. E. Nelson, E. I. Fulmer and R. Cessna, Journal of Biological Chemistry, volume 46, page 77, 1921; M. B. MacDonald and E. V. MeCollum, Journal of Biological Chemistry, volume 46, page 525, 1921; K. H. Coward and J. C. Drummond, Biochemical Journal, volume 15, page 530, 1921; W. B. Bottomley, Annals of Botany, volume 34, pages 345 and 353, 1920.)