GROWTH OF PLANTS 



125 



The explanation of growth-promoting substance, or growth 

 hormone, has advanced especially rapidly during recent years, 

 chiefly in connection with the work of the Dutch physiologist 

 Went and his coworkers, and of Cholodny in Russia. To obtain 

 the growth hormone in quantities sufficient for obvious display 

 of its activity. Went applied the following procedure. The cut 

 tips of oat and corn coleoptiles (the tips of these two plants are 

 the most active) about 1 to 2 mm. in length were placed on a thin 

 lamina of agar gel and remained thus for about 1 hr. After this, 

 the lamina was cut into separate small cubes, each of which 

 contained a certain amount of the hormone (Fig. 30). Such 

 blocks, when placed eccentrically on decapitated seedlings, 



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Fig. 30. — Method of extracting and measuring auxin. 1, plate of agar with 12 

 coleoptile tips; 2, tips removed, agar divided into 12 cubes; 3, coleoptile with 

 incision; 4, 5, decapitated coleoptiles; 6, first leaf partially pulled out; 7, one of 

 the agar cubes placed on one side of the coleoptile; and 8, result, bending of the 

 coleoptile. 



provoke a growth curvature. The amount of hormone sufficient 

 to produce a. curvature of 10 deg. in an oat coleoptile was defined 

 as 1 "oat unit." Such blocks impregnated with the growth 

 hormone were used for studying some of its properties. It 

 proved to be a fairly stable substance, not destroyed by boiling. 

 The rate of its diffusion through agar-gel was used to determine 

 its molecular weight, which proved to be about 350. 



Growth hormones may be formed not only in tops but also in 

 several other parts of the plant organs. Thus according to the 

 experiments of Cholodny, the substance is formed in definite 

 elements of the phloem in hypocotyls of lupines and sunflowers. 

 If by means of a special borer the central cylinder with all its 

 elements is removed from a cut seedling, the growth rate is 

 appreciably inhibited. Now if the cut tip of a corn coleoptile is 



