72 GROWTH HORMONES IN PLANTS 



chymatous tissue has been corroborated (Went, 1928a; Laibach 

 and Kornmann, 19336). 



Polarity of Trans-port. — The movement of growth substance in 

 the Avena coleoptile has been found to be a polar phenomenon, 

 occurring only in a basipetal and not in an acropetal direction. 

 Beyer (1928o) showed that the transport of growth substance 

 from a decapitated and unilaterally replaced tip into the coleop- 

 tile stump was inhibited if a ring segment of coleoptile was 

 inverted and placed between the tip and the base. If the inserted 

 coleoptile ring was oriented normally, however, the growth 

 substance migrated downward freely. Went (1928a) reached a 

 similar conclusion in regard to the polar nature of movement: 

 agar blocks were placed on the two terminal cut surfaces of 

 coleoptile cylinders, 2 mm. in length (Fig. 2) (Went). The upper 

 block contained growth substance, and the lower one was plain 

 agar. After some time, both blocks were analyzed for growth 

 substance in order to determine the amount of transport. Very 

 little growth substance was present in the lower block when the 

 coleoptile cylinders were in an inverted position, showing that 

 growth substance did not migrate from an agar block placed at the 

 morphological base to a block placed at the morphological apex. 

 Van der Weij (1932), using a similar method, found a significant 

 amount of transport in inverted coleoptile cylinders 1 mm. 

 long, but this was thought to be due to diffusion of the growth 

 substance in the water adhering to the coleoptile. In agreement 

 with other investigators, the general conclusion was reached that 

 probably movement of growth substance can take place only 

 basipetally through living tissues. It was found, in addition, 

 that growth substance can be transported against a gradient, 

 i.e., from a place with a lower concentration to another with a 

 higher concentration. If an agar block placed at the mor- 

 phologically basal end contained more growth substance than a 

 block placed at the apical end, the concentration was soon dimin- 

 ished in the upper block, although an increase in growth substance 

 in the lower block could not be shown. A concentration at the 

 basal end of a coleoptile cylinder ten to twenty times that at the 

 apical end does not impede basipetal transport. Gravity appears 

 to have only a very slight effect upon the direction of movement, 

 which takes place just as well upward as downward in a mor- 

 phologically basipetal direction. 



