234 TRANSLOCATION IN PLANTS 



but they seem to have no influence on transport of fluorescein. Analytical 

 data clearly demonstrate a marked reduction in carbohydrate transport 

 through petioles chilled to a temperature between and about 4°C. That 

 transport is reduced at these temperatures, that the effect appears rather 

 abruptly, and that the critical temperature appears to be somewhere around 

 6°C. is indicated by these transport studies as well as those on bud inhibition. 

 Some have failed to duplicate the temperature effects, but it seems that the 

 material was kept too close to the critical temperature. Although transport 

 takes place through living cells, it has not been positively established that 

 these cells take an active part in the transport or whether they are merely 

 passive tubes. Final understanding on this point is linked with an under- 

 standing of the mechanism of transport. The universality of speciaUzed 

 sieve-tube structures containing living protoplasm in all plant tissues show- 

 ing rapid transport would seem to point to a mechanism that requires living 

 protoplasm to take an active part in transport. The protoplasmic stream- 

 ing hypothesis seems at present best adapted to meet the requirements of 

 transport and to demand the structure and conditions that obtain in phloem 

 tissues. 



