THE ANGIOSPERMAE 



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form of imbrication corresponds to sl ^ phyllotaxy, in which part i is the 

 only one overlapping at both edges, and part 5 the only one which is com- 

 pletely overlapped, while parts 2, 3 and 4 are both overlapped and over- 

 lapping. This is sometimes regarded as typical imbrication. 



Fig. 1 1 24. — Diagrams of aestivation. A, Vah'ate. B, Im- 

 bricate. C, Contort or Rotate. D, Quincuncial. 



The common phenomenon of circumnutation of the vegetative apex 

 shows clearly that the rate of growth may vary in different radial sectors of 

 the axis and this is, no doubt, true of the floral axis also, since irregularities 

 commonly arise through delay or acceleration in the growth rate of indivi- 

 dual sectors. These inequalities may, in some cases, lead to the deformation 

 of the receptacle or even to zygomorphy of the whole flower, but more 

 commonly they lead only to departures from the typical quincuncial 

 imbrication of the petals, due to the edges of petals slipping over or under 

 those of their neighbours, as they expand. 



The extreme case of departure from quincuncial arrangement is that in 

 which each petal, in succession, overlaps its up-spiral neighbour and is 

 overlapped by its down-spiral neighbour. This is called contort or con- 

 volute and it is characteristic of certain families, such as Gentianaceae. 

 Both right-handed and left-handed convolution and imbrication occur and 

 reversal is not uncommon. In Saxifraga graniilata the existence of right- 

 handed and left-handed races has been detected (Fig. 1125), an interesting 

 case, in which not only the imbrication of the petals, but the order of opening 

 of the flowers in the inflorescence and of the opening of stamens in the an- 

 droecium, follow spirals which are reversed in the two races. Compton 

 showed reason to believe that reversal of the overlap of the margins in the 

 coleoptile of Barley was due purely to chance, but that can scarcely apply to 

 so extensive a change as in the above example, where a genetic difference 

 between the races seems to be probable. Analogous reversals in the spiral 

 shells of Gastropods are well known. 



