226 TRANSLOCATION IN PLANTS 



The temperature at which he kept the petiole ranged from 

 5 to 7°C., which was almost certainly above the critical 

 point at least for a part of the time, and this, I am sure, 

 accounts for the lack of great differences. It is perhaps 

 surprising that if materials are carried by streaming proto- 

 plasm, there is not more retardation at intermediate tem- 

 peratures, for the rate of streaming must, it seems, be less 

 at 8 than at 18°C. As pointed out in my earlier paper, 

 however (Curtis, 1929), some factor other than rate of 

 streaming may be hmiting the amount of transport so 

 that the rate of streaming does not become important 

 until the lower temperatures are reached. Furthermore, 

 these findings bear only on the amount of material trans- 

 ported, not on the velocity of transport. 



Recent experiments of Schumacher (1930) give addi- 

 tional evidence that living cells are actively engaged in 

 transport. By the use of eosin, which is toxic and which 

 seems to have a specific effect on sieve tubes, inducing 

 a formation of callose plugs on the sieve plates, he was able 

 to interfere with translocation through the phloem. He 

 claims this work proves conclusively that transport takes 

 place almost exclusively through the sieve tubes because 

 they alone develop the callose plugs, while the companion 

 and parenchyma cells appear normal and show normal 

 movements. Although the plugging of the sieve tube is 

 clear and positive evidence, the evidence is not clear that 

 the activity of the companion and phloem parenchyma 

 cells is not also interfered with. Even though streaming 

 continued in parenchyma cells, the ability of the proto- 

 plasm to absorb or carry normal solute may have been 

 destroyed by the eosin. If the stoppage of translocation 

 is due solely to plugging of the sieve tubes, the evidence 

 does not directly support the contention that living cells 

 take an active part in transport. It would be interesting 

 to know if local chilUng or anesthetics cause a temporary 

 coagulation of the contents or the development of a callose 

 plug. Dr. Knudson has brought to my attention the fact 

 that temperatures below 11°C. may cause stoppage of 



