ORDER OP APPEARANCfi OP FLORAL ORGANS. 169 



in a single longitudinal section. Comparison of tlie.su figures 

 gives a clear idea of the sequence in which the organs make 

 their appearance ; namely, in acropetal succession, — that is, 

 the younger or newer are always nearest the extremitj*. 



471. According to Paj'er, the sepals alwaj's precede the petals, 

 the petals the stamens, and the stamens the pistils, in time of 

 appearance. But in a few cases, of which Lysimachia is one, 

 it may happen that a given circle of organs is somewhat de- 

 la3'cd in forming ; for instance, in the figures the stamens are 

 seen as considerable protuberances before the petals are clearly 

 outlined. This fact has been considered by some to indicate 

 that the corolla in such cases consists of an intercalated whorl 

 between two other whorls already somewhat developed. But a 

 careful examination of Lysimachia and most other cases shows 



rather that the petals or the corolla-lobes are laid down in their 

 proper sequence, but that they are temporarily outstripped by 

 the sepals and the stamens. 



The appearance of the forming flower when seen in vertical 

 section is shown in Fig. 130, and a perspective view is given 

 in Fig. 125, exhibiting the late-appearing petals and the much 

 larger stamens. 



472. Since the several organs of the flower are modified 

 leaves symmetrically arranged on an axis, the histological con- 

 stituents of a leafy branch will be found in the flower, albeit 

 much modified in some of their characters. These constituents 

 are, (1) a framework of flbro-vascular tissue, upon which is 

 extended (2) parenchyma, covered by (3) epidermis. 



Fig. 130. Lysimachia quadrifolia. Longitudinal section through a flower in which 

 the corolla is just appearing. The elevation on the right has been cut tljrough exactly 

 in the median line, while that on the left has been cut on its edge. Letters the same as 

 in Fig. 129. (Pfeffer.) 



