THE METHOD OF MOVEMENT 217 



influenced by temperature and that transport is very slow 

 at temperatures approacliing 0°C. Van der Wey, on the 

 other hand, finds the velocity of transport of the growth 

 substance to be independent of temperature, although the 

 amount transported in unit time is influenced. These 

 differences may be related to the fact that one was moving 

 through sieve tubes and the other through parenchyma 

 cells. Schumacher finds fluorescein is carried solely in 

 protoplasm and does not permeate the wall excepting along 

 plasmodesma, while Van der Wey suggests, but without 

 much supporting evidence, that the growth substance is 

 carried by wall plasm that in part permeates the walls. 

 Sugar transport almost certainly is not polar and salt 

 transport probably is not, while movement of the growth 

 substance seems strictly polar. It is possible, however, 

 that movement may have fixed polarity in coleoptiles and 

 be reversible in other parts. The amount of the growth 

 substance carried is obviously much less than that of 

 carbohydrates, nitrogen, or other nutrient elements; for 

 the initial concentration of growth substance used in most 

 of his experiments was usually of the order 0.000001 

 normal, or 0.00003 per cent. A mechanism that is ade- 

 quate for the transport of the growth substance, there- 

 fore, may not be adequate for transport of sugars. 



The velocity of transport of the growth substance seems 

 much slower than that of fluorescein, but this may be due 

 to the fact that rates for growth substance seem to have 

 been determined with parenchyma tissues, while the higher 

 rates of movement of fluorescein were observed with sieve 

 tubes. The data presented by Van der Wey indicate a 

 transport of the growth substance of from 0.05 to 0.25 mm. 

 per minute, while Schumacher estimated fluorescein to be 

 carried through sieve tubes at about 5 mm. per minute. 

 In hair cells, on the other hand, he found fluorescein to 

 move about 0.05 mm. per minute. The velocity of normal 

 sugar transport is not known. It is true that Miinch 

 and Crafts have estimated velocities of flow of phloem 

 exudate, but both the rate as well as the mechanism of flow 



