VII GROWTH OF SPECIFIC ORGANS 8ll 



[d) Interrelations between buds; integration of the plant 



On a typical dicotyledonous shoot there is a lateral hud in the axil of every 

 leaf. Usually most of these do not develop, or develop only very slowly, but if the 

 growing apex is killed or removed, a few of them develop at once. Such behavior 

 implies that these lateral buds had been capable of development at all times, but 

 were inhibited by the growing apex. 



More than twenty years ago, in the "exploratory" phase of the study of plant 

 growth hormones, this inhibition was shown to be exercised by auxin coming 

 from the apical bud. If the apex was removed, then the lateral buds could be 

 maintained in the inhibited state by applying auxin. Supplementary inhibitions 

 are exercised by the much smaller amounts of auxin coming from the leaves in 

 whose axils the lateral buds lie. (See Went and Thimann, 1937; and Soding, 1952, 

 Chap. 1 1 for reviews of the earlier experiments.) 



It is evident that this system provides a major hormonal means for the integra- 

 tion of the growth of the plant. Its effectiveness varies, of course, from one plant 

 to another. At one extreme are plants like the sunflower with a single axis and 

 complete suppression of the lateral buds, a clear example of "apical dominance". 

 At the other extreme are the much-branched dwarf composites with little or no 

 dominance, bearing numbers of branches, both lateral and direct from the root 

 crown, which develop simultaneously. Most dwarf plants, indeed, branch much 

 more freely than their normal relatives. Monocotyledons are no exception, for 

 in the grasses and cereals the lateral bud is the "tiller," of which one to several 

 develop in a season and may bear additional heads ; the development of these, 

 which is greatest in dwarf forms, is also inhibited by auxin and released upon 

 weakening or removal of the auxin source (Leopold, 1949). 



A feature of the inhibition is that it is dependent on continuity of the auxin 

 supply. Thimann and Skoog in 1934 found that they had to apply fresh auxin 

 in place of the terminal bud every 6 h. in order to maintain complete inhibition ; 

 a gap of 1 2 h. between applications led to development of the buds. Recently 

 Libbert ( 1 954a) has observed that, if the terminal bud of Pisurn is removed and 

 subsequently replaced intact, the break in auxin supply is enough to allow 

 lateral buds to grow out, even though the terminal bud afterwards becomes grafted 

 back in place. The growing lateral may then come to inhibit the further growth 

 of the terminal. Correspondingly, any treatment interfering even slightly with the 

 transport of auxin, or even a temporary fall in the rate of auxin formation (Hat- 

 cher, 1948) may cause lateral buds to develop. 



In a way the suppression of one meristem by another can be considered a general 

 phenomenon. Btinning (1952, 1957) has proposed that all meristems fragment 

 into smaller ones, called "meristemoids", which can only become active when 

 removed from the inhibiting action of the parent or from one another. The for- 

 mation of stomata and roothairs are offered as examples. Inhibition of the develop- 

 ment of stomata by one another is responsible for the even pattern of their spacing 

 on the leaf. The formation of stomata can also be inhibited by auxin through 

 causing normal cell division of the epidermis instead (Biinning and Sagromsky, 

 1948). Is was noted in section Vllb, too, that root tips inhibit the formation of 

 lateral roots. It would be natural, therefore, to consider lateral bud apices as 



Literature p. 8i6 



