364 ANNUAL REPORT SUHTHSONIAN INSTITUTION, 1940 



all appear to be involved in the utilization of carbohydrates in the 

 respiration of the living cell. It would seem likely therefore, that 

 there is considerable similarity among the mechanisms by which this 

 fundamental vital process is performed in plants and in animals. 

 It is possible, of course, that those roots which do not appear to re- 

 quire an external supply of a given growth factor are able to synthe- 

 size the substance from simpler nutrients which are supplied. 



It is very interesting that in the case of thiamin, a mixture of the 

 two portions of the molecule^ is just as effective as the complete 

 thiamin molecule itself. The root appears capable of synthesizing 

 the vitamin from these building blocks. Furthermore, the vitamin 

 Bi requirement of the tomato root can be met by supplying only 

 the thiazole portion ; this species seems to have the capacity to manu- 

 facture the pyrimidine portion necessary to complete the molecule. 

 The pea root, on the other hand, is unable to do this. 



Excised roots have been used also to study the synthesis of vita 

 min C, or ascorbic acid, in the root. The production of vitamin C 

 in intact plants appears to be intimately connected with the chloro- 

 plasts and yet the vitamin is of very widespread occurrence in the 

 roots and other nongreen parts of such plants in which chloroplasts 

 are lacking. The question arises whether it is actually manufactured 

 in the roots or is transported to them from the shoot. Now, excised 

 moonflower roots develop chlorophyll if cultured in light but not if 

 grown in darkness. Although they make good gi'owth in the dark, 

 their content of vitamin C does not increase. In light, however, the 

 ascorbic acid content increases 400 to 1,000 percent. From this find- 

 ing, it seems probable that the colorless roots of intact plants do not 

 synthesize vitamin C but receive their supply from the green tops. 

 It should be mentioned that there is, as yet, no direct evidence that 

 this vitamin plays a role in the vital economy of the root. 



In recent years there has accumulated a large body of evidence 

 which indicates that the growth and behavior of various organs of 

 the plant are regulated by growth-promoting substances or hor- 

 mones. The problem of the relationship of growth substance to 

 root growth shows a number of similarities to the vitamin C prob- 

 lem. Growth substance seems to be present in all roots so far 

 investigated but, curiously enough, concentrations of growth sub- 

 stance, or auxin, which stimulate the growth of shoots are inhibitory 

 to roots. Root elongation may possibly be somewhat enhanced, how- 

 ever, by extremely dilute growth substance solutions (1 part in 10 

 billions to 1 part in 100 billions). One would like to know whether 

 the growth substance is produced in normal intact roots or whether 



» The vitamin Bi, or thiamin, molecule can be split Into two fractions, one consisting of 

 a thiazole, the other of a pyrlraldlne grouping. 



