604 



WEED FLORA OF IOWA 



FLOWERS. 



A satisfactory definition of a flower has not yet been agreed 

 upon by botanists. For this reason it seems better to describe a 

 flower rather than attempt to define it. 



Fig. 457. Complete flower; cal, calyx; cor, corolla; p, pistil, and s, stamens. 



(After Thome.) 



A complete flower consists of four cycles or sets of organs — sepals, 

 petals, stamens and pistils. The sepals taken together constitute 

 the calyx; the petals taken together constitute the corolla. 



Since the cells of the stem most active in forming new organs lie 

 in the tip, it seems most natural that the organs appearing last, 

 would be at the stem tip and that the age of organs would increase 

 as their distance from the stem tip. According to this scheme, the 

 succession of floral sets would be sepals, petals, stamens and pistils. 

 This is probably the order of succession in many flowers, but many 

 exceptions have been found. In shepherd's purse (Capsella) the 

 petals are last to appear, while in the dandelion and other com- 

 posites the sepals are last to appear. 



Fig. 458. Diagrammatic cross section of a perfect flower. St, ovary showing 



two cells ; S, stamens ; B, corolla ; K, calyx. 



(After Thome.) 



