210 TRANSLOCATION IN PLANTS 



different mechanism or that there is a simultaneous flow in 

 both directions. In the latter case it would seem that the 

 surface-active substance causing the flow would move in 

 one direction and other substances would be passively 

 carried in the opposite direction, perhaps in the resulting 

 stream of moving protoplasm. Possibly the electrical 

 charge or the molecular configuration or solubility of 

 different substances will determine which of the moving 

 streams they will enter and thus determine the direction 

 of their movement. 



38. Relations between the Transport of Viruses and 

 Hormones and That of Foods. — Studies on the transport of 

 viruses may be of considerable assistance in solving normal 

 translocation problems; for with but few possible excep- 

 tions (Grainger, 1933) it has been rather clearly established 

 that natural transport of viruses is restricted to living 

 tissues and rapid movement takes place in the phloem. 

 Bennett (1927) describes experiments which clearly indicate 

 that transport of the leaf curl virus of raspberries takes 

 place through the phloem, and that the movement is 

 unidirectional and in the direction of the major transport 

 of food. That is, in plants with well-formed leaves, the 

 transport is downward only, while in new shoots, drawing 

 their food supply from diseased roots or the older diseased 

 canes, the transport is upward. Growing shoots that had 

 appeared above the surface of the soil and were probably 

 sending foods back to the roots failed to become infected 

 from the roots, even though the shoots continued growth 

 and were susceptible to direct inoculation. 



On the other hand Caldwell (1931) working with the 

 virus producing yellow mosaic of tomato demonstrated 

 simultaneous movement in both directions and at approxi- 

 mately equal rates. Later (1934) using Johnson's No. 1 

 mosaic of tobacco and the aucuba or yellow mosaic of 

 tomato, he found movement throughout both tobacco and 

 tomato plants to be independent of the position of inocula- 

 tion and independent of the direction of major transport 

 of foods. The virus spread throughout the plants at 



