DEVELOPMENT OF PLANTS 



445 



very common occurrence. These plants with tlieir small white 

 or yellow flowers aggregated in conspicuous inflorescences are 

 among the most familiar weeds of waste places, fence rows and 

 barren fields. The flowers are of a more specialized type than 

 in the open blossoms of the buttercups and poppies, and present 

 a variety of interesting devices that can only be hinted at in this 

 lesson. More commonly, perhaps, the stigmas are receptive as 

 soon as the flower opens and the anthers are still closed, thus 



Fig. 309. Examples of the mustard family, order Papaverales: .1, branch 

 of Brassica, one of the turnips. B, a flower enlarged. C, flower in section. 

 D, a petal, showing narrow claw and broad blade. E, pistil. F, fruit, the 

 valves of the two carpels opened to show the seeds. 



ensuring crossing. The short stamens are more freciuenth' util- 

 ized only for crossing since they are below the stigma and so 

 placed as to be in line with the nectar glands that are often located 

 in sac-like enlargements of the perianth. The four long stamens 



