218 TRANSLOCATION IN PLANTS 



of exudate may be abnormal. Mason and Maskell (1928a) 

 found an increase in sugar content of the bark of cotton, at 

 a distance of about 50 cm. from the leaves, to lag behind an 

 increase in the leaves by about 2 hr. This would indicate 

 a linear transport at a rate of about 25 cm. per hour. 

 Reported rates of transport of viruses which were supposed to 

 be moving largely through phloem tissues have ranged from 

 0.25 mm. per minute and lower up to 25 mm. per minute. 

 In many of these cases the calculated rates are not very 

 exact because the total time between inoculation at one 

 point and arrival at another was included, and there was 

 no way of knowing the time at which the virus entered 

 the phloem. Some have used leaf hoppers for introducing 

 the virus directly into the phloem and have thus a fairly 

 accurate knowledge as to the time of entrance of the virus. 

 By such a method Bennett (1934) has observed rates of 

 movement up to 25 mm. per minute for transport of virus 

 in sugar beets. 



If movement through the sieve tubes of all substances 

 being transported is unidirectional, this would point to a 

 mechanism of the sort proposed by Miinch. However, 

 the movement of more than one substance in one and the 

 same direction, such as virus and sugar, for example, does 

 not mean that both are being carried by the same mechan- 

 ism. Sugar may be carried along a diffusion gradient 

 where the movement is being hastened by streaming, while 

 the virus, because of its electrical charge, may be carried 

 cataphoretically or may attach itself only to that part 

 of the protoplasm which is moving in the one direction. 

 It is even conceivable that in special cases there may be 

 unidirectional flow, as in a leaf or flower which is being 

 emptied of certain materials before abscission, where the 

 protoplasm itself may be actually withdrawing through 

 the sieve tubes. Kienitz-Gerloff (1891) has made a similar 

 suggestion. Such a withdrawal may be comparable to 

 the withdrawal of the Plasmodium of a myxomycete from 

 the medium through which it had spread itself. Such a 

 withdrawal could take place by a mechanism very different 



