THE ANGIOSPERMAE 



1127 



side and the development of parts on the higher side is correspondingly 

 accelerated. The adaxial stamens are nearly fully formed before the abaxial 

 petal appears. Even a flower which is ultimately actinomorphic, e.g., 

 Parnassia, may show this bilateral asymmetry in early stages of its develop- 

 ment and is thereby seen to be cryptozygomorphic. Papilionate flowers 

 of the Leguminosae show a similar unilateral development but in the 

 reverse direction, from the abaxial to the adaxial sides. 



It can be readily understood that in flowers where there is partial sup- 

 pression of certain organs, such as the sterilization of part of the androe- 

 cium, the reduced parts lag behind the normal parts in their growth and 

 usually stop before arriving at the mature form. They may also, however, 

 be initiated later, as in Sderanthus annuiis (Fig. iioi), where the two fertile 



Fig. iioi. — Scleranthiis anuitits. Floral ontogeny. A and F, Early and nearly mature stages 

 of the whole flower. The three sterile stamens have undergone chorisis. B, C, D and E, 

 Successive stages in the development of the ovary. Lettering as in Fig. 1099, ot; = ovary. 

 {After Payer.) 



Stamens are initiated considerably earlier than the three sterile stamens, 

 which remain much smaller than the others. The subdivision of a primor- 

 dium may lead to a similarly differentiated development, as in PhiladelpJnis 

 (Fig. 1 102), where each of the four original rudiments divides radially 

 into 7-9 segments. The median segment, or the median three segments, 

 are so much more forward than the others (some of which abort altogether) 

 that they press inwards and appear to form, in the mature flower, an inner 

 whorl of stamens, although all the primordia formed a single whorl. 



Lastly, there is the important phenomenon of protogyny, the carpels 

 maturing before the stamens. This, which is due to the enhanced rate of 

 development of the upper zone of the receptacle as a whole, is apparently 



