212 PLANT PHYSIOLOGY 



bring about these changes is still unknown, but Denny (1926) 

 found that the dominance of the apical bud in potato tubers can 

 be checked by thiourea; in place of only one, two to five buds 

 then develop from each eye. It has been suggested that hormones 

 change the permeability of the protoplasm thus causing changes 

 in the osmotic conditions inside the cell. Others have proposed 

 that they cause a separating out of the constituents of the proto- 

 plasm, much as freezing might, and in this way aid in secretory 

 activity. The complete chain of chemical reactions still remains 

 to be worked out ; only a few links have thus far been forged. 



Vitamins.— Of all the substances in this category which have 

 attracted the attention of the public and of physiologists alike, 

 the vitamins are most conspicuous. This is because they have 

 been found to play such an important part in animal nutrition. 

 Funk (1911), Osborne and Mendel (1913-1915), as well as many 

 other investigators including Sir Frederick Hopkins, have shown 

 that animals cannot live and thrive on pure foods. In addition 

 to the foods and ordinary mineral salts which animals must have, 

 there must also be present for normal growth and development 

 certain " impurities," to which Funk has given the name vitamins. 

 These vitamins are not foods but food accessories, which have 

 specific stimulating or regulating effects and thus determine the 

 normal physiological operation of the organism. Comparatively 

 little is yet known about their chemical nature, with the exception 

 of vitamin D, but this little is sufficient to tell us that the name, 

 which implies the presence of an amine group, was unfortunately 

 chosen since the majority of vitamins contains no such groups. 



At the present time six of these vitamins have been especially 

 studied and inasmuch as two of these have been definitely added 

 to the list during the past five years, it is probable that all have 

 not yet been found. These vitamins, called A, B, C, D, E, and G 

 are made for the most part only by plants, with the result that 

 animals are dependent upon plants for them. They (animals) do 

 not have the power to make their vitamins from foods which lack 

 them. Fish oil, for example, which is rich in certain vitamins, has 

 the same odor as the oil of diatoms, which play a large part in the 

 diet of fish. The absence of any one of the vitamins causes a certain 

 characteristic disease known as a deficiency disease (avitaminosis), 

 which with the corresponding vitamins will now be briefly de- 

 scribed. 



