THE TRANSLOCATION OF AUXINS 577 



is translocated acropetally (base to apex) through the coleoptile from the 



seed (Skoog, 1937)- 



Chemical Nature of the Auxins. — In a brilliant series of investigations 

 beginning in 193 1 Kogl and his co-workers have succeeded in isolating from 

 biological sources three chemically pure crystalline substances vi^hich give all 

 the reactions of auxins when tested by the oat coleoptile technique. These 

 three substances have been called auxin a (C18H32O5), auxin b (CigH3o04), 

 and heteroauxin (CioHgOoN). The chemical names of these substances 

 are auxentriolic, auxenolonic, and indole-3-acetic acid, respectively. All three 

 of these compounds are monobasic acids. Auxin a has been prepared in the 

 pure state from human urine, and both auxins a and b have been isolated from 

 malt and various vegetable oils. Hexeroauxin has been isolated from 

 urine and from certain yeasts and molds. This compound (indole-3-acetic 

 acid) has also been prepared synthetically. 



More recently it has been discovered that a number of other compounds 

 also result in curvature of oat coleoptiles when tested by the usual technique 

 and by some investigators all such compounds are classed as auxins. Most of 

 these compounds are not as effective in inducing this response as the three 

 described above, and it has not been shown that any of these compounds occur 

 naturally in plants. The physiological effectiveness of auxins a and b and 

 heteroauxin is almost unbelievable. It has been calculated that i mg. of 

 either auxin a or auxin b, applied in agar blocks, can cause a curvature of 10 

 degrees in 50,000,000 oat coleoptiles. Heteroauxin is about half as effective. 



The question of which is the naturally occurring auxin in various species 

 of plants has not yet been settled with finality. Present indication are, how- 

 ever, that auxin a is the naturally occurring auxin in oats and other higher 

 plants while heteroauxin is of common occurrence in fungi. 



The Translocation of Auxins. — The transport of the auxins from one 

 part of a plant to another presents some distinctive and interesting features. 

 If a block of agar containing auxin is affixed to the morphologically upper 

 end of a segment of oat coleoptile, and a block of pure agar to the lower 

 end, auxin will move into and accumulate in the lower block. The final con- 

 centration of auxin in the basally attached block may greatly exceed that in 

 the one affixed to the apex (Fig. 128, A). If the position of the coleoptile 

 segment between the agar blocks is reversed, i.e. if the block containing auxin 

 is affixed to the morphologically basal end no translocation of auxin will occur 

 (Fig. 128, 5). Translocation of auxin in oat coleoptiles apparently takes place 

 through the parenchyma tissue. The results of such experiments show: (l) 

 that translocation of auxin in the oat coleoptile is polar, i.e. occurs only basi- 



