AUXINS AND GROWTH OF THE OAT COLEOPTILE 573 



A 



A 



A 



A A 



A 



A 



and may nearly regain the original rate (Fig. 124). Retipping the coleoptile 



with a short segment cut out of another coleoptile somewhat below the apex 



results in little or no increase in growth rate. Such experiments indicate that 



the growth of a coleoptile, which occurs in the more basal regions, is mam- 



tained only under the influence of some 



sort of a "stimulus" originating in the 



tip, whence it is transmitted basipetally 



(apex to base) through the coleoptile. 



Furthermore, since at the age in their 



development at which coleoptiles are 



used in such experiments cell divisions 



have ceased, this influence is exerted on 



the enlargement phase of growth. 



Went (1928, 1935) placed the cut- 

 off tips of oat coleoptiles on a thin layer 

 of 3 per cent agar and after one hour 

 removed them and cut the agar into a 

 number of equal-sized small blocks (Fig. 

 125). If one of these blocks was placed 



upon the stump of a decapitated coleoptile, the rate of elongation was ac- 

 celerated just as if the stump had been reheaded with a fresh coleoptile tip. 

 On the other hand, retipping a coleoptile with a block of pure agar had no 

 appreciable accelerating effect on elongation. It seems evident from these 

 results that some substance or substances was transported out of the tip into 



B c 



B c 



B c 



Fig. 124. Effect of removal of tip 

 on elongation of oat coleoptile. {A) 

 check, {B) tip severed and replaced, 

 (C) tip removed. The effect of treat- 

 ment is shown by differences in in- 

 crease in length of coleoptiles at right. 



Fig. 125. Diagram illustrating stages in the collection of auxin in agar from 



coleoptile tips. 



the agar block, and subsequently out of the block into the tip of the decapi- 

 tated stump, whence it was translocated downwards to the elongating region 

 of the coleoptile (Fig. 126). The substances which induce such a response 

 are now classed as auxins. 



Many other plant organs such as stems, petioles, flowerstalks, and coleop- 

 tiles of other species behave similarly upon removal of the apical region. 

 Elongation is stopped or retarded by such a treatment, but will be resumed 



