THE METHOD OF MOVEMENT 165 



therefore suggested that the space occupied by this water 

 in the walls might act as a channel through which the 

 solution is forced. Assuming all this water of the walls 

 filled a single tube, he calculated that a pressure gradient 

 of only 6.1 X 10~^ atmosphere per centimeter would be 

 adequate to cause a flow at the rate observed. Of course, 

 the space occupied by water cannot behave as a single pore 

 but the pressure gradient calculated is sufficiently small 

 to allow for considerable correction for increased resistance. 

 His calculations, he suggested, allow for an increased 

 resistance of about 4,400 times that of the single pore. 



When the cut is first made the material is exuded from 

 the phloem at a linear rate of 1 cm. per minute or faster. 

 This flow is rapidly reduced but on repeatedly cutting thin 

 sections from the end it approaches a rather steady rate 

 of around 0.3 cm. per minute or 18 cm. per hour. "Calcu- 

 lations show that this is approximately the rate that would 

 be required to deliver the volume of solution necessary 

 to form the fruits of these plants. Since rates of this 

 magnitude cannot be accounted for on the basis of flow 

 through the sieve tubes, it is suggested that the solution 

 has passed through the phloem walls." Similar rates 

 varying between 13.5 to 19 cm. per hour through the walls 

 of the phloem alone, he estimated, would account for the 

 observed rates of transport of carbohydrates from the leaves 

 of Tropaeolum majus and Phaseolus multiflorus, as well as 

 the materials necessary to account for the observed sea- 

 sonal increase in dry weight of the stem of Bartlett pear. 



Since other mechanisms that had been proposed seem 

 inadequate and since actual flow from cut phloem was so 

 rapid as to seem to preclude the possibility of flow through 

 the sieve pores, Crafts first suggested (1931) that the walls 

 of phloem cells constituted the major path of transport. In 

 his second paper (1932) he suggested that transport takes 

 place through both walls and lumen. By the time he 

 wrote the third paper (1933) he had come to realize the 

 high resistance to flow through walls and suggested that 

 the lumen is the major path. He suggested further that 



