252 THE VITAMINS 



plants become food for the group of small sea organisms known as 

 plankton and this in turn for larger species up to the crustaceans and 

 small fishes on which codfish feed. In such ways the synthesis of vita- 

 min A in the small green plants of the sea presumably becomes the 

 source of the abundant supplies of this substance which we find in 

 cod-liver oil ; the cod, it will be remembered, having very little fat else- 

 where in its body. 



In 1922, Wilson published experiments indicating that etiolated 

 wheat shoots showed evidence of a synthesis of vitamin A at this stage 

 in the development of the young plant. 



In 1927, Coward extended her studies of the problem of the develop- 

 ment of vitamin A in the sprouting seed and young growing shoot. She 

 concluded that "seedlings of wheat which have germinated for two 

 days and have only made three rootlets and no shoot or only a very 

 short one, do not contain any more vitamin A than similar ungerminated 

 seeds." And, on the other hand that: "Etiolated shoots of wheat and 

 white and yellow corn may contain small quantities of vitamin A when 

 grown to a height of about 10 to 12 cm. above the ground." 



Hence there may be a slight development of vitamin A during the 

 growth of the etiolated shoot ; but it is certain that the main synthesis 

 of vitamin A in the plant takes place only under the influence of light. 

 In nature, of course, this means ordinary sunlight with its mixture of 

 rays of various wave-lengths. Such evidence as is thus far available 

 seems to indicate that the short ultra-violet rays are neither essential 

 nor deleterious to the synthesis and accumulation of vitamin A in the 

 normal living plant tissue. Coward (1927) concluded that radiation 

 from a mercury-vapor quartz lamp is effective; but also, that in the 

 presence of an abundance of visible radiation the addition of ultra- 

 violet rays has no influence on the ultimate amounts of vitamin A 

 contained in the plant tissues. "Within reasonable limits, excessive 

 illumination of the living plant tissue does not seem to have any deleteri- 

 ous eflfect on the vitamin A formed in it." 



In a study of the persistence of vitamin A in plant tissues, Coward 

 (1925a) found that vitamin A is not used up in any process carried on 

 by living plant tissue in the dark; that the vitamin A content does not 

 decrease when a leaf loses its green color and becomes yellow; that it 

 is completely destroyed when the leaf dries up, becomes brown, and 

 dies ; and that it does not diffuse appreciably into water from the cut 

 ends of shoots. 



Morgan and Smith (1928) found the vitamin A content of tomatoes 

 to increase during ripening. 



