THE METHOD OF MOVEMENT 227 



bleeding from latex tubes of banana fruits. In a later 

 paper Schumacher (1933) gives strong evidence that the 

 living protoplasm in sieve tubes is essential for the trans- 

 port of fluorescein. , 



The available evidence seems rather conclusive that 

 translocation takes place through the phloem only as long 

 as it is alive. Although experiments on chilling, anes- 

 thesia, etc., indicate that the cells must be not merely 

 alive but take an active part in the transport, this evi- 

 dence is not incontrovertible and this phase needs further 

 investigation. 



Final understanding as to the part played by living cells 

 is closely linked with that of the mechanism of transport. 

 A mechanism of streaming protoplasm is clearly dependent 

 upon living and active cells. A mechanism of the sort 

 suggested by Van den Honert (1932) would probably be 

 dependent on living protoplasm in so far as living proto- 

 plasm is necessary for maintaining the active interface 

 over which the materials are carried. In a sense, the 

 mechanism may be considered as a special type of proto- 

 plasmic streaming in which the interfacial protoplasmic 

 films move faster than the more granular visible proto- 

 plasm. A mechanism of the sort suggested by Miinch 

 seems independent of the activity of living cells, except 

 as they may be concerned in furnishing the osmotic mem- 

 brane at the source, in maintaining a gradient, in keeping 

 the tubes in condition for transport, and in preventing 

 leakage from the system. A mechanism of the sort 

 proposed by Crafts seems entirely independent of living 

 cells except for the introduction and removal of the sugars 

 at the two ends of the system. According to the proposal 

 of Crafts, the phloem must also be surrounded by living 

 cells or something to prevent leakage. With such a 

 mechanism a system of open or dead tubes more like that 

 of the xylem would seem to be more effective. 



The very facts that sieve tubes seem universally present 

 in all types of plants in which rapid transport of solutes 

 takes place, that they are more extensive and most highly 



