THE METHOD OF MOVEMENT 177 



less than 1 cm., the rate of transfer might be greater than 

 over the shorter distance where movement is hmited to 

 diffusion. The steepness of diffusion gradients between 

 supplying and receiving cells alone, therefore, may be of 

 minor importance in determining solute distribution. If 

 there is a continuous series of active streaming cells between 

 supplying and receiving cells and if there is actual flow 

 through sieve pores, as seems likely, then the distance 

 between the regions becomes a minor factor. 



Points favoring the hypothesis that transport through 

 the phloem takes place by a combination of diffusion with 

 protoplasmic movement of the nature of rotation or 

 streaming are as follows: (a) Such movements could 

 account for transport at a rate immensely faster than 

 can be accounted for by diffusion alone, ih) Such a 

 mechanism would allow for simultaneous transport in 

 both directions, and considerable evidence points toward 

 such simultaneous transport, although for specific sub- 

 stances the movement may be almost exclusively in one 

 direction (see Sec. 38). (c) It could also account for the 

 selective transport of special substances to special tissues 

 (see paragraph 4 of Sec. 28). {d) The postulated mech- 

 anism could easily account for transport to particular 

 tissues where, because of the distance from the supply or 

 because of only slight diffusion or pressure gradients, other 

 suggested mechanisms would seem not to be adequate, (e) 

 This mechanism also seems to offer a simple explanation 

 of the mutual interchange of material between a pair of 

 active organs such as root and leaf or root and shoot (see 

 Sec. 42). (/) This hj^Dothesis does not demand great 

 pressure differences which are indispensable for the hypoth- 

 eses of Miinch and of Crafts, {g) It could also account 

 for ready transport to a receiving tissue independently 

 of whether that tissue had a higher or a lower osmotic 

 concentration or turgor pressure than the supplying tissue. 

 It could thus account for an emptying of flaccid storage 

 tissues into turgid receiving tissues as well as for a rapid 

 transport from leaves during the day when their turgor 



