76 GROWTH HORMONES IN PLANTS 



piration stream. The dead cells of the xylem are doubtless the 

 pathways of transport; hence this observation does not contradict 

 the evidence for polar movement through liviyig tissues. Charac- 

 teristic bending, proliferation, and rooting responses result 

 from the upward movement of growth substances through stems 

 (Fig. 41). Absorption and movement were delayed or prevented 

 under conditions of low transpiration. Movement was observed 

 in both directions through dead tissue. 



The Mechanism of Growth-substance Transport. — Any 

 attempt to formulate an explanation of the mechanism of growth- 

 substance movement should explain the rate as well as the 

 polarity of transport in agreement with the results reported for 

 the Avena coleoptile. A satisfactory explanation must be in 

 agreement with the transport phenomena observed in other plant 

 organs. Several possibilities will be examined. 



Diffusion. — That all growth-substance movement cannot be by 

 diffusion is clear from the magnitude of the rate of transport. 

 Went (1928a) has computed that two hundred times more growth 

 substance is moved in a given time than could be accounted for by 

 diffusion. Then, too, the conclusions of van der Weij concerning 

 the amount of transport at higher temperatures are not com- 

 patible with the conception of movement as a process of diffusion. 

 If diffusion were concerned, the amount moved would have to be 

 inversely proportional to the length of the coleoptile cylinder 

 and directly proportional to the concentration of the original 

 solution. Experimental evidence has shown that this is not the 

 case in the Avena coleoptile. 



The theories offered in explanation of the translocation of 

 nutritive materials in plants do not suffice for movement of 

 growth substance in the Avena coleoptile. The movement of such 

 materials is not polar and is bound up with certain conduct- 

 ing cells which may not enter into the question of growth-sub- 

 stance transport. Certainly too little is known yet to make a 

 final statement regarding the role of diffusion in the movement of 

 growth substance. 



Protoplasmic Streaming. — Since the time of de Vries (1885) it 

 has been accepted generally that in many cases the movement 

 of dissolved substances is facilitated by protoplasmic streaming. 

 It is possible, therefore, that streaming plays a part in the trans- 

 portation of growth substance. Brauner (1922) suspected that 



