PLANT-TISSUE CULTURES — ^WEINTRAUB 365 



it is supplied to them from the shoots. A number of growth-sub- 

 stance studies with excised roots have been carried out but, unfor- 

 tunately, the results have been conflicting so that a definite answer 

 cannot yet be given. Since it is established that the excised root 

 can grow indefinitely without any external supply of growth sub- 

 stance it must be concluded that if auxins are indeed necessary for 

 root growth they are manufactured by the root itself. In this 

 connection it is of interest that the excised root does not lose its 

 capacity for responding to the force of gravity, a property generally 

 assumed to be related to the action of growth hormones. The geo- 

 tropic response of excised corn roots cultured in a solid agar medium 

 is shown in plate 1. The position of the tubes in plate 1, left, was 

 altered at intervals so as to change the direction of the gravitational 

 stimulus; the tubes shown in plate 1, right, were maintained in a 

 vertical position throughout. 



One of the first uses to which excised root cultures were put 

 was the maintenance of stocks of plant-disease viruses. The roots 

 of tomato were used for the cultivation of the viruses of aucuba 

 mosaic and of tobacco mosaic. Such cultures have the advantage of 

 being free from accidental contamination with other viruses, of 

 requiring comparatively little space, and of permitting close control 

 of environmental and nutritional conditions. It is interesting that 

 the infected roots did not exhibit any symptoms of disease although 

 the virus multiplied actively in them. 



Work along somewhat similar lines was undertaken several years 

 ago by Lewis and McCoy in an attempt to induce nodulation by root- 

 nodule bacteria on excised legume roots. Although some nodules 

 were produced by Rhizobium on excised roots of the black wax bean, 

 the results were not very satisfactory, owing possibly to unfavorable 

 culture conditions. The use of the tissue-culture technique would 

 appear to be of great value for the study of many problems related 

 to the physiology of nitrogen fixation by root-nodule bacteria since 

 the nutritional and environmental conditions can be so easily and 

 so precisely manipulated. In view of the great strides in technique 

 made since the above-mentioned experiments were performed, a re- 

 newed attack appears to offer considerable promise. 



Of the various applications to which excised roots have been put 

 perhaps the most striking is the study of root pressure.* In these 

 experiments the basal end of a single excised tomato root was in- 

 serted into a capillary manometer and the root with manometer was 

 then returned to a flask of nutrient solution (pi. 2). In this way 



*A report of this work, for wbich the $1,000 prize of the American Association for the 

 Advancement of Science was awarded to Dr. Philip White in 1937, appeared in the Annual 

 Report of the Smithsonian Institution for 1938, pp. 489-497. 



