THE TRANSPORT OF GROWTH SUBSTANCES 73 



Rate and Amount of Transport. — Van der Weij (1932, 1934) 

 also carried out experiments dealing with the rate and the 

 amount of growth-substance transport and with the effect of vari- 

 ous external factors upon the magnitude of these phenomena. 

 The experimental technique was essentially like that of Went 

 (1928a). Coleoptile cylinders provided at the upper end with 

 agar blocks containing growth substance were placed erect upon 

 agar that contained none. Increase of concentration in the lower 

 block was investigated over a period of time. The original con- 

 centration in the upper blocks was measured by the angle of 

 deviation that they could produce when placed unilaterally on 

 decapitated coleoptiles; the concentration found in the lower 

 blocks was given in percentage of the original. Since the results 

 of these experiments were usually portrayed graphically, a 

 complete estimate of the experiments is not always possible. 

 According to van der Weij, the capacity of transport may 

 be considered as the amount of growth substance found per unit 

 of time in the lower agar block after equilibrium has been reached. 

 This value was determined from the curves that were produced 

 when the increase of growth-substance concentration in the lower 

 block was plotted as a function of the time. The rate of trans- 

 port is the distance through which the growth substance moves 

 per unit of time. In order to determine this value, the exact 

 moment at which the growth substance starts to enter the lower 

 block must be ascertained. The period of time required for the 

 substance to traverse the known length of the coleoptile and first 

 appear in the lower block was determined from the point of 

 intersection of the concentration curves on the time axis. 



Close inspection of van der Weij's curves gives the impression 

 that some of the extrapolations are arbitrary. For example, it 

 would be possible to conclude that the rate of transport decreases 

 wdth an increase in the temperature; but, on the other hand, from 

 experiment 126 (1932), it might be concluded that it increases 

 with rising temperature. The points of the curve fluctuate so 

 much that the point of origin of the curve cannot be determined 

 with certainty. Hence it is not definitely established that the 

 rate of growth-substance transport is independent of the 

 temperature. 



Rate of Transport. — The conclusions that may be dra^^^l 

 from van der Weij's experiments are significant, nevertheless. 



