176 TRANSLOCATION IN PLANTS 



to carry a similar amount a distance of lO^i (more nearly 

 the thickness of the cell wall) would be 3.19 X 10"^^ ^j^y, 

 or about 0.003 sec, ten billion times as fast. Of course, one 

 would not expect so steep a gradient as this in conducting 

 tissues, but it is evident that difTusion over short distances 

 is extremely rapid. The large number of strands traversing 

 the sieve plates or those connecting one living cell with 

 another through plasmodesma, although they offer but a 

 small total cross-sectional area, should be highly effective 

 in allowing movement from one cell to another by diffusion, 

 as was pointed out by Brown and Escomb (1900). 



A rotating stream of the type postulated could readily 

 carry salts, proteins, or any soluble or adsorbed material 

 from the roots in one direction in the same cells that are 

 carrying sugars in the opposite direction from the photo- 

 synthetic organs. In such a system the direction of 

 movement of a given solute would be determined by 

 concentration differences* between the region of supply 

 and that of use, and the amount of solute moved would be 

 determined by the combined influence of concentration 

 difference, the rate of streaming, the number of cross 

 walls, and perhaps also the effectiveness of the strands in 

 preventing mixing of the streams going in opposite direc- 

 tions. It is conceivable, therefore, that movement might 

 be more rapid between two points a decimeter apart than 

 that between two points a centimeter apart, when the 

 actual concentration difference is the same in both systems. 

 For, if the elongated cells connecting the receiving and 

 supplying regions over the longer distance all show active 

 streaming, whereas the protoplasm in the cells over the 

 shorter distance is not streaming; then, since the total 

 cross-wall thickness through which diffusion must take 

 place over the decimeter distance would probably be much 



* These concentration differences might appear as actual positive gradi- 

 ents, or, where living cells are capable of concentrating certain solutes or 

 ions against a concentration gradient, they may appear negative when 

 comparing the receiving cell or tissues and the source of supply. Taking 

 into consideration the actual dynamic equilibrium conditions, however, the 

 gradient along the path should always be positive. 



