II26 



A TEXTBOOK OF THEORETICAL BOTANY 



of those which are opposite the petals, and alternate with the sepals. The 

 latter may therefore surpass the former in growth and displace them, pro- 

 ducing the appearance of an outer whorl standing opposite the petals. This 

 is one of the ways in which obdiplostemony may arise, as we have explained 

 in a previous section (see p. 1095). 



Another departure from strict succession is seen in the late development 

 of the petals in many cases. Their primordia may originate before those of 

 the outer stamens but they lag behind them in further development and 

 may only complete their growth after all the stamens have been completely 

 formed. If the petals have been indeed evolved from sterilized stamens, 

 this lag may be regarded as a check due to the " new path " along which they 

 develop, in contrast to their fertile companions. 



Those zygomorphic flowers in which the zygomorphy is due to a depar- 

 ture from the normal radial symmetry of the receptacle, show irregularity 

 in the development of their parts, especially the petals, whereby one side 

 of the flower is in advance of the other. In Reseda, which shows this dif- 

 ference in a marked degree (Fig. 1 100), the side of the receptacle nearest to 

 the inflorescence axis is, from a very early stage, higher than the opposite 



Fig. 1 100.— Reseda odorata. Floral ontogeny. A and B, Two stages of whole flower. C and 

 D, Two stages of the gynoecium, older than in A and B. Lettering as in Fig. 1099, 

 except that in C, p = placenta. {After Payer.) 



