GROWTH AND GROWTH HORMONES IN PLANTS 395 



is, of course, the basis of the art of pruning, which consists essentially of 

 removing auxin-producing centers so as to allow the growth of buds that had 

 previously been inhibited. 



Like all general phenomena, bud inhibition or apical dominance is subject 

 to numerous variations. Some of these shed valuable light on the process, 

 others are merely puzzling. For example, there is a bud in the axil of each 

 of the cotyledons of a seedling. In spite of the wealth of nutrient available 

 in these cotyledons, the bud does not develop, a fact which was observed 

 by Goebel and other early students of growth but could not be explained 

 by them in terms of the nutritional concepts which at that time were uni- 

 versally used. We now know, of course, that the growth of these buds is pri- 

 marily controlled by the inhibiting action of auxin rather than the promoting 

 effect of the nearby nutrient supply and that the auxin comes both from the 

 terminal bud and from the cotyledon itself. Hence removal of either of these 

 sources usually promotes growth of the lateral bud. Recently, however, there 

 have been described a few cases where removal of the cotyledons actually 

 slows down the growth of the buds in their axils, suggesting that supply of 

 materials can easily become a limiting factor too. 



At the other extreme there are, in some bushes, lateral buds close to the 

 apex which, instead of being inhibited by the apex, develop strongly. The 

 extent to which this occurs is greater the more vigorous the growth of the 

 main shoot. Evidently these "anticipatory shoots," as Champagnat has called 

 them, are in some way released from the auxin inhibition. 



An interesting variant on the theme is the formation of short shoots, 

 common to many fruit trees and notable in the woody gymnosperms. In 

 apple and other fruits the short shoot is the flowering shoot, or "fruit-spur," 

 but in Larix or Ginkgo, for instance, it may be purely vegetative and, indeed, 

 always is in young trees, since these do not fruit. If the terminal bud and part 

 of the stem of a young Ginkgo tree is removed, two or three of the short 

 shoots become long (though sometimes this only happens in the next season), 

 while if auxin is applied to the decapitated stem they remain short. It seems, 

 therefore, that the lateral short shoot is a bud that is inhibited by auxin only 

 so far as elongation is concerned, but that, unlike an ordinary lateral bud, it 

 can open and allow its leaves to grow to full size. 



Somewhat similar to the inhibition of lateral-bud development is the in- 

 hibition by auxin of another important trait in plant behavior, namely the 

 dropping ("abscission") of leaves or fruits in the autumn or when sufficiently 

 aged. In many plants, if the blade of any full-grown leaf is removed, the 

 petiole will drop off a few days later, and this makes it convenient to study the 

 process. Abscission results from growth of some cells of special properties 

 at the base of the petiole. If auxin is applied, growth of these cells is inhibited, 

 and hence the petiole stays on more or less indefinitely. Similarly by spraying 



