8l2 PLANT GROWTH lO 



''meristemoids". A field of inhibition around each leaf and bud primordium in 

 the apical meristem has been envisaged in a similar way (see e.g. Wardlaw, 1952, 

 especially Figs. 1 7 and 18 therein). Unfortunately the absence of concrete evidence 

 for such a meristem inhibitor, other than auxin itself, limits the usefulness of the 

 hypothesis at present. 



A variant on the typical inhibition of lateral buds is provided by the "short 

 shoot," a branch or axis which does not elongate but in which the leaves do develop, 

 forming a rosette. Somewhat analogous is the behavior of biennial plants, which 

 form a rosette in the first year and elongate in the second. Within the rosette, each 

 leaf by its auxin production partially, but never completely, inhibits the next in 

 order. In a few cases, notably the biennial Hyoscyamus, elongation of the short 

 shoot is brought about by gibberellic acid and this may be followed by flowering 

 (Lang, 1956). Or the normal requirement for light to cause elongation may be 

 set aside by supplying gibberellic acid (Curry and Wassink, 1956). Thus, failure 

 of the short shoot to elongate in these plants may be due to deficiency of gibberel- 

 lins or physiologically similar growth factors. 



In woody plants, especially in the larch (Larix) and the maidenhair (Ginkgo 

 biloba), the short shoots will elongate if the part of the branch above them is re- 

 moved ; it is not enough to remove the apical bud alone. At least in young Ginkgo 

 plants, if such decapitation is followed by application of auxin paste to the cut sur- 

 face, the short shoots can be completely prevented from elongating (Gunckel et al., 

 1949), though their leaves open normally. The short shoot bud is thus comparable 

 to a lateral bud in which avixin inhibits the elongation but does not inhibit un- 

 folding and expansion of the leaves. The necessity for removing part of the stem 

 in order to relieve the inhibition receives its explanation from the absence of strict 

 localization of the auxin production ; this is not limited to the apical bud and in- 

 deed not even centered there, but in the growing long shoot most of the auxin is 

 produced within the upper internodes of the stem itself (Gunckel and Thimann, 

 1949). Exactly the same situation appears to prevail in long shoots of the plum 

 (Hatcher, 1948). It may be connected with the development of a rib meristem. 



In another short shoot system, that in Cercidiphyllum, an ornamental Japanese 

 bush, the relations appear more complex, though they are the same in respect of 

 the control over shoot elongation. Decapitation leads to outgrowth of many of the 

 short shoots into long ones (providing they are not flower-bearing), while applica- 

 tion of auxin to the cut surface keeps the shoots short (Titman and Wetmore, 1955) : 

 in this respect it is like Ginkgo. It differs, however, in that the terminal bud, to- 

 gether with the uppermost internode, abscisses in midsummer, when its auxin 

 production is still high. The explanation for this odd behavior is probably that 

 the lateral short shoot buds actually destroy auxin in the stem, for when they are 

 removed the amount of auxin which difTuses from the stem cut surface increases, 

 and correspondingly the terminal internode stays on. Apparently, this is a rare 

 case of integrative action based on auxin destruction. 



If the short shoots are lateral bvids whose inhibition is only partial, the "anti- 

 cipatory shoots" are lateral buds which are free from inhibition altogether. These 

 are shoots derived from large buds close to the apex in several woody plants, es- 

 pecially birches (Champagnat, 1954 and earlier notes) and they develop when the 



