GROWTH OF SPECIFIC ORGANS 



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20 25 30 35 

 Days after pollination 



Fig. 12. "Waves" of formation of free auxin in the 

 achenes of the strawberry (data of Nitsch, 1950) and 

 in the grains of corn (data of Stehsel, 1950, unpub- 

 hshed). Auxin determined by ether extraction and 

 bioassay, without liberation of any in "bound" 

 forms. (Replotted from Nitsch, 1952.) 



auxin for some 3 to 4 weeks after fertilization, then a "wave" of auxin production, 

 demonstrable by ether extraction on successive days, coincides with the endo- 

 sperm becoming cellular (Luckwill, 1953). A second and larger wave, reaching 

 its peak some 10 weeks after fertilization (the amount and the time depending 

 on the variety of apple), was ascribed to the growing embryo itself, though it 



may also be due to the seed-coat. 

 Similar "waves" of auxin forma- 

 tion occur in strawberries, corn 

 and other fruits (Fig. 12). In the 

 apple variety Lane's Prince Al ber t 

 some of the embryos abort, and 

 this corresponds with even greater 

 auxin production, presumably by 

 the endosperm. Finally the fruit 

 tissue, at least in apple and pear, 

 is found to contain another, and 

 a relatively unstable, auxin. It is 

 when this auxin begins to disap- 

 pear that the peduncle of the fruit 

 begins to form an abscission layer 

 {cf. p. 798) and hence the fruit soon 

 falls. If the fruits are sprayed with 

 NAA or 2,4-D at this point, ab- 

 scission can be delayed for some 

 weeks, and this practice is now general with orchardists. 



Thus the auxin may be supplied from as many as 4 diflferent sources during the 

 growth and life of the fruit (assuming one may condense the partial findings on 

 several different fruits into a single composite picture). The supply from the em- 

 bryo is doubtless the most important, for if that part of very young crookneck 

 squash fruits which contains the ovules is removed, growth virtually ceases. This 

 growth could be reinstated by applying indolebutyric acid (Gustafson, 1 951, and 

 earlier work there cited). Similarly in the strawberry, where the "achenes" (seeds 

 in their carpels) are on the outside and thus available to the experimenter, it is 

 clear that if they are removed all growth stops (Nitsch, 1950). If achenes are re- 

 moved on only one side, growth on that side stops ; hence the auxin supplied to the 

 receptacle does not diffuse far. The receptacle appears to form no auxin itself, 

 while the achenes produce it in a single "wave" with its steep peak about 12 days 

 after fertilization (Nitsch, 1955; see Fig. 12). 



There is evidence that these successive auxin sources may supply different sub- 

 stances. That in the first wave in the apple and in the black-currant is probably 

 lAA (Luckwill, 1954). The auxin in the apple seeds, has been thought to be the 

 ethyl ester (Teubner, 1953). The strawberry achenes contain 4 other auxins besides 

 lAA (Nitsch, 1955). 



Analyses of developing bean pods showed that there is a "wave" in them at 

 about 8 days from fertilization of a substance differing from auxin in being trans- 

 ported upward in test plants and causing excessive stem elongation (Mitchell et al., 



Literature p. Si6 



