376 



PLANT MORPHOLOGY 



Here zonation involves the three outer floral sets. In epigynous flowers 

 the upward growth of the receptacle involves the ovary as well as the other 

 floral parts, so that the ovary is embedded in the receptacle and the sepals, 

 petals, and stamens seem to arise from its summit. In hypogynous and 

 perigynous flowers the ovary is superior, while in epigynous flowers it is 

 inferior. Hypogyny represents the most primitive and most common 

 condition, epigyny the most advanced. Perigyny is intercnediate and 

 least common. 



In perigynous and epigynous flowers the structure surrounding the 

 ovary and bearing the sepals, petals, and stamens on its rim is generally 



ABC 



Fig. 320. Diagram.>s illustrating hypogyny (.4.), perigyny (B), and epigyny (C). In each 

 flower the receptacle is stippled. 



regarded as a zonal upgrowth of the receptacle because, in the develop- 

 ment of the flower, such an upgrowth actually occurs. Another view, 

 supported by evidence from vascular anatomy, is that the structure 

 referred to is made up of the fused basal portions of the sepals, petals, and 

 stamens, to which, in epigynous flowers, the carpels are also united. 

 Although such fusion cannot be seen in floral development (ontogeny), it 

 is assumed to have occurred during the course of floral evolution. 



Floral Development. A longitudinal section through a very young 

 flower bud reveals the fact that the floral parts arise at the tip of the 

 receptacle as rounded protuberances of meristematic tissue. They arise 

 in much the same way as foliage leaves from a vegetative stem tip. Ordi- 

 narily their appearance is acropetal, the sepals coming first, next the 

 petals, then the stamens, and finally the carpels. This sequence is shown 

 in the buttercup (Ranunculus), for example, a primitive flower that is 

 apocarpous and hypogynous and one in which the stamens and carpels 

 arise in spiral succession on an elongated receptacle (Fig. 321). As in all 

 flowers, the apical meristem does not continue to grow indefinitely, but 

 sooner or later becomes transformed into carpels. 



The usual order of appearance of floral parts is modified in certain 

 flowers, especially where one set is being suppressed. In the shepherd's- 

 purse iCapsella), one of the Cruciferae, the petals appear after the other 



