DEVELOPMENT OF PLANTS 467 



on the outside of the umbel become somewhat irregular, owing to 

 the unequal enlargement of the outermost petals. These small 

 flowers that individually cannot be seen at a distance of a few feet 

 become the most conspicuous feature of our fields owing to their 

 association in large numbers into compact, lace-like umbels. The 

 aggregation of flowers has been noticed in various groups, as in 

 the mustards, various familifes of the Rosales, in the Sapindales 

 and buckthorns, of which latter simple regular type the carrot 

 flower would appear to be a natural sequence (compare Figs. 

 321, G; 324, D). But in no group has so successful and varied 

 an arrangement of the flowers been achieved. The advantages 

 of this umbellate arrangement are apparent. The crowding of 

 the flowers is attended with the reduction in their size and in 

 saving of material while they have gained in the number of 

 flowers and in conspicuousness. This arrangement also increases 

 the chances of seed production since the insect in a single visit 

 may cross a score of flowers in crawling over the umbel. Autog- 

 amy is prevented at first by the difference in the maturation of 

 the anthers and stigmas, frequently one or more days interven- 

 ing between these two conditions ; and in many genera the flowers 

 are imperfect, the stamens and pistils being located on differ- 

 ent parts of the umbel and so further assist in crossing. But 

 even if the great variety of small lapping insects that swarm over 

 these flowers were absent the construction of the umbel is such 

 that the flowers are usually able to effect pollination unassisted. 

 The variety of devices for the accomplishment of this work is 

 without parallel in any other group. The opening of the flowers 

 of an umbel proceeds either from the circumference towards 

 the center or from the center outwards. In some genera, as 

 Eryngium, the flowers on the margin open first and the stigmas 

 are ready for crossing while the anthers are closed and bent 

 down upon the petals. On the following day, the inner adjoining 

 set of flowers is in the same condition while the anthers of the 

 marginal flowers have been lifted up by their filaments so that 

 they reach out and come in contact with the stigmas of the ad- 

 jacent inner flowers, the curvature of the styles often assisting in 

 bringing the two organs together. In this way, ample oppor- 



