9z 



THE STRUCTURE OF ECONOMIC PLANTS 



In many instances, the individual members of a given whorl do 

 not arise simultaneously but in succession. This is illustrated in 

 the papilionaceous Leguminosae (Pisum), where the abaxial sepal 

 arises first, followed by the two lateral ones; and, finally, the 

 two adaxial lobes. Similarly in the corolla, the two petal primor- 

 dia which form the keel are the first to arise, followed by the two 

 lateral primordia forming the wings; and, lastly, by the primor- 

 dium of the standard. In other cases, it has been demonstrated 



A B 



Fig. 33. A, the floral branch of Spiraea; B, a young flower showing perigyny. 



that the sequence in a given cycle may be progressive in a counter- 

 clockwise or a clockwise spiral. 



Many exceptions to the acropetal mode of development are 

 known. In certain Compositae, the much reduced or modified 

 sepals do not appear until after the differentiation of the stamens 

 and carpels; and in some of the Cruciferae, the petals are the last 

 primordia to appear. Instances have been observed in the Rosaceae 

 in which the succession of development is sepals, inner stamens, 

 carpels, outer stamens, and petals; while in other members of this 

 family, the primordia of the carpels appear before the full quota of 

 stamens has been determined. This indicates that there are excep- 

 tions to true acropetal succession and that the actual appearance 

 of the floral organs may be in part basipetal and in part acropetal 

 or irregular. In some cases, these apparent irregularities can be 

 interpreted as a delayed enlargement of potential primordia or a 

 non-divergence of parts of a cycle or adjacent cycles. For example, 

 in the Primulaceae the petals appear late in ontogeny after the 



