Occurrence and Role of Growth Hormones 81 



in auxin transport. Exhaustive studies of transport through the non- 

 vascular parts of the coleoptile, as compared with the vascular elements, 

 demonstrated little or no difference provided only that physiological 

 concentrations of auxin were used. In stem and pedicel tissues the 

 phloem appears to be the most active in auxin transport (Cooper, 

 1936), but apparently any live metabolizing tissues are able to trans- 

 port auxin. 



The most striking characteristic of auxin movement is its generally 

 strict basipetal polarity. This can be studied most readily in short 

 sections of stems or coleoptile tissue, to which a supply of auxin is 

 provided in an agar block at the physiological apex of the piece and 

 from which the transported auxin is collected in an agar block at the 

 physiological base. Such an experiment will give one a measure of 

 basipetal or downward transport. Repetition of the same procedure 

 with the stem or coleoptile section inverted will give one a measure of 

 acropetal or upward transport. By such experiments as these, van der 

 Weij (1932) found that auxin is transported exclusively basipetally in 

 the Avena coleoptile. This is true regardless of the position in 

 which the tisstie is held during the experiment; the transport is al- 

 ways toward the physiological base of the tissue. The same strict 

 polarity of auxin transj)ort is true of many stems (van der Weij, 1933; 

 van Overbeek, 1933; I'himann and Skoog. 1931; Clooper, 1936) and 

 leaves (Avery, 193.5). 



A striking demonstration of the inflexibility of the polar trans- 

 port mechanism can be seen in the polarity of stem cuttings of Tagetes 

 which were rooted in soil in an inverted position. Even though they 

 were rooted and grown in the inverted position, they would transport 

 auxin only to their old physiological base for 36 days, at which time 

 new conductive tissues seem to have been established with the new 

 polar orientation (Went, 1941). 



The strict downward transport of the growth hormone in plants 

 is a classic instance of polarity in a biological system and affords an 

 excellent opportimity for a study of polarity in general. 



A strict polarity of auxin movement apparently does not always 

 exist, for Leopold and Guernsey (1953) have shown that in the coleus 

 plant the basipetal polarity becomes weaker and weaker as the distance 

 from the vegetative stem apex increases (figure 40). Furthermore, in 

 flowering stems there is some acropetal movement even at the stem tip. 

 They have produced some evidence that a substance is formed in the 

 flowering stem apex which permits acropetal transport of auxin. 



The movement of auxin in roots can take place either acropetally or 

 basipetally (Heidt, 1931) although in at least some cases a strict polar- 



