VII 



GROWTH OF SPECIFIC ORGANS 



813 



o lAA 



Fig. 14. Diagrams of some bud inhibition experi- 

 ments. A, Libbert, 1954 {cf. also Bagda, 1956). 

 B, Thimann, 1937. C, Bagda, 1956. 



stem is still growing rapidly. Auxin diffusion determinations show that those 

 apices which have anticipatory shoots nearby actually produce more auxin than 

 those without such buds. It is always the most vigorously growing shoots that are 

 concerned. 



The polarity of auxin transport means that the inhibited buds are normally 



below the source of auxin. But 

 such polarity is evidently not abso- 

 lute. Several workers have shown 

 that small amounts of auxin can 

 be transported in stems from 

 base towards apex (see review of 

 Thimann, ig54b). Also the trans- 

 port can be interfered with by such 

 substances as triiodobenzoic acid 

 and 2,4-D (Kuse, 1953; Nieder- 

 gang-Kamien and Skoog, 1956; 

 Hay, 1956b) and as a result apical 

 dominance can be weakened or 

 even prevented, axillary buds 

 developing beneath the block in 

 transport just as though the apical 

 bud had been removed. Whether 

 naturally occurring substances in 

 the stem similarly modify polarity 

 has not been proved, but the appearance of such polarity-modifying material has 

 been reported in stems of Coleus when they enter the flowering state (Leopold and 

 Guernsey, 1953). Buds have several times been observed to be inhibited by a source 

 of auxin acting in the direction opposite to auxin polarity. A recently described 

 example is the inhibition exerted by lilac leaves on the development of axillary buds 

 situated above them on the stem ( Champagnat, 1 95 1 ) . Long ago Snow observed that 

 in pea plants with two equal shoots growing from the base, the axillary buds on 

 one shoot were under (rather slight) inhibition from the terminal bud of the other. 

 Now Libbert (1954a) and Bagda (1956) have shown that indoleacetic acid re- 

 placing the terminal bud of one shoot can exert clear inhibiting influence on such 

 a distant bud situated up a side branch (Fig. 14A). Evidently polar transport in 

 branched plants is not nearly as strict as was imagined. But if the shoots are un- 

 equal the polarities are unequal too, for auxin applied to the shorter of two shoots 

 does not inhibit buds on the longer one (Fig. 14C; Bagda, 1956). In general, how- 

 ever, it is clear that such anti-polar inhibition is not evidence against the role of 

 auxin, as it has so often been interpreted to be, and thus the position of auxin as 

 the inhibiting and therefore integrating agent is better estabUshed than before. 



However, to explain how a growth hormone can inhibit the growth of lateral 

 shoots while promoting that of the terminal shoot is no easier now than when it 

 was discovered 22 years ago. Many of the theories were critically reviewed in 

 detail by the writer in 1939 and it is unnecessary to go over them again here. One 

 possibility is simply to deny that auxin could be the inhibitor, as has been attempted 



Literature p. 816 



