242 TRANSLOCATION IN PLANTS 



protoplasmic streaming of the tissues leading to A is thus 

 increased while that of the tissues leading to B is less 

 active, then, other things being equal, solutes should be 

 carried faster to A, thus allowing for continued greater 

 activity of the growing tissue of the fruit. This greater 

 activity would allow for more rapid transfer of solutes 

 though the concentration of solutes at this receiving tissue 

 were equal to, or even greater than, in B. Thus, although 

 there may be actually a smaller difference in concentration 

 between the source and the receiving tissue in A than 

 between the source and that in B, so that the seeming 

 diffusion gradient would favor B, the more active streaming 

 in the conducting cells leading to A would tend effectively 

 to reduce the distance through which movement is limited 

 to diffusion, or would tend to increase the steepness of the 

 diffusion gradient by more quickly carrying the solute 

 from one end of the cell to the other. Thus, as was 

 explained in Sec. 33, the effective diffusion gradient over a 

 distance of a decimeter may be steeper than over a distance 

 of a centimeter if in the former case streaming were active 

 in the conducting cells and diffusion limited to occasional 

 cross walls, while in the latter case movements were 

 limited entirely to diffusion, or if the rate of streaming was 

 very much reduced. The mechanism of transport pro- 

 posed by Miinch would not explain movement into A but 

 would favor movement to B. 



That a diffusion gradient alone accompanied by proto- 

 plasmic streaming of either the visible sort or a faster 

 moving interfacial film does not fully account for transport 

 into organs, such as these hypothetical fruits, is indicated 

 by the fact that transport from the fruits that fail to 

 receive food continues to take place, and therefore a trans- 

 location mechanism must still be active. This is indicated 

 by the observations of Howlett (1926) that nitrogen is 

 removed from the weaker fruits which later abscise. 

 Schumacher's (1931) observations on transport of nitrogen 

 from darkened leaves is a comparable case, as is also the 

 emptying of leaves before leaf fall (Deleano and Andreesco, 

 1932). 



