THE ANGIOSPERMAE 1159 



to be an addition to the calyx itself, from those in which they appear to form 

 a distinct structure. 



That the sepals are themselves of bract nature is borne out, as we have 

 shown above, by the triple vascular trace which supplies them. They are 

 also known to produce, occasionally, axillary buds, as in the small white 

 flowers of hopyrum thalictroides (Ranunculaceae) which has petaloid sepals 

 and no petals. Secondary, pedicellate flowers may spring from the axils 

 of its perianth members or alternatively sessile, incomplete flowers, stand- 

 ing in the sepal axils, may be included within the perianth of the primary 

 flowers. 



The corolla, which consists of the inner series of perianth parts in 

 complete flowers, is generally the most conspicuous portion of the flower, 

 usually distinctively coloured and delicate in texture. Delicate as they may 

 seem, many corollas possess, however, a remarkable resistance to desic- 

 cation. Stomata are usually present, but, as the guard-cells contain no 

 chloroplasts, they may not be functional. 



The epidermis is often cuticularized, sometimes heavily, or covered by 

 a thin, waxy coat. The flowers of many species, notably those of some 

 Malvaceae, still appear fresh after having been detached and left on a table 

 for twenty-four hours. Other species, although similar in appearance, begin 

 to wilt almost immediately after they are cut. Comparatively little is 

 known about the physiology of these and other characteristics of petals. 

 We have referred to the question of the withering of flowers previously in 

 this chapter, on p. 1152. See also Chapter XXV, p. 1480. 



It is in the corolla that the character of the flower is most prominently 

 displayed. We have already pointed out that two main classes are readily 

 recognizable, namely those in which the petals are all free from each other 

 (apopetalous or polypetalous), and those with the petals coherent to 

 each other (sympetalous), but both these classes include the most extreme 

 variations of form and size. Petals may, of course, be completely absent 

 and the flower be therefore apetalous, or they may be transformed into 

 nectaries, of which we shall give some examples later. Transitions between 

 petals and stamens are not uncommon, particularly in spiral flowers such as 

 Trollius, where a zone of indecision may appear exhibiting every gradation 

 between perfect petals and perfect stamens, in a continuous series. 



The anatomy of sepals shows unifacial structure at their points and that 

 of petals at their bases, in which the latter agree with stamens. We may 

 conclude from this that a sepal is morphologically the equivalent of a 

 phyllopodium or leaf base and that a petal is the equivalent of the upper 

 leaf portions. The two sets of organs are thus of distinct nature, though 

 both fundamentally referable to the leaf category. If the petal-stamen 

 homology is sound then this reference to the leaf category would hold also 

 for the stamens. 



There is a strong probability, as we have shown above in considering 

 receptacular anatomy, that the petals and stamens are of the same morpho- 

 logical nature, and the balance of probability is in favour of regarding the 



