DISTRIBUTION OF INHIBITORS IN PLANTS 409 



toplasm is continuous or separated by plasma membranes. There is some 

 evidence that both situations occur. In the first place, the movement of 

 many substances in phloem is much more rapid than could be accounted 

 for by simple diffusion; a factor of 40,000 may be observed (Biddulph and 

 Cory, 1957). If many membranes had to be passed, it is evident that they 

 could not possess the usual permeability properties of the plasma membrane; 

 instead, they would have to participate actively in the transport. Further- 

 more, various substances move at different rates indicating that neither sim- 

 ple diffusion nor mass flow of fluid is the primary mechanism. Metabolism 

 is necessary for normal transport through the phloem. Part of this energy 

 may be utilized for protoplasmic streaming (cyclosis) which has been 

 thought by some to be an important factor in accelerating the movement of 

 dissolved substances: however, cyclosis cannot account for either specificity 

 of transport or the rapid rates observed. Gross movement of fluid through 

 phloem can occur, perhaps as the result of active processes, but dissolved 

 substances usually move more slowly than the water, as in a chromatogram. 

 The observations of polarized transport (i.e., movement of a substance in 

 one direction from the site of application) would imply active participation 

 of membranes. Studies with fluorescein have demonstrated the complexity 

 of the transport. The rates of movement are different in various regions 

 of the plant and the polarity may also differ; for example, application of 

 the dye to stems results in upward movement into leaves or fruits, while 

 application to a basal region results in movement downwards into the roots 

 (Crafts, 1951). 



Foliar painting or spraying of a substance may result in the distribu- 

 tion of the substance throughout the plant. This may be seen most clearly 

 with dyes. If a potato plant is dug up. the apex of a leaf excised, and the 

 cut surface immediately immersed in eosin solution, within an hour all the 

 leaf veins are stained and some dye has penetrated into the roots; in several 

 hours the dye has entered the tuber itself. Similar results have been ob- 

 tained with P^2^ C^*-labeled photosynthesates, S^^, tritiated water, and some 

 herbicides. When P^- is applied to a single unifoliate leaf of a bean plant 

 for 2 hr and the plant is then allowed to grow for a week, radioactivity is 

 demonstrated throughout the plant (Barrier and Loomis, 1957). Many 

 factors have been shown to influence absorption and translocation: the air 

 humidity, light intensity, age of the leaf, wetting of the leaf, and whether 

 absorption is from upper or lower surface of the leaf (Thorne, 1958). The 

 influence of light on translocation may be through the sugars produced, 

 since application of sugar with the transportable substance will usually 

 allow normal translocation to occur. The application of C^^-a-methoxy- 

 phenylacetate to leaves led to diffusion of this sul)stance from the roots 

 into a bathing solution within 5 hr: this increased steadily for 4 days and 

 then declined until bv the tenth day exudation was negligible (Linder, etal., 



