DEVELOPMENT OF PLANTS 



433 



contains many familiar native and cultivated plants which may be 

 recognized by the hollow internodes of the stems, leaves variouslv 

 lobed and attached by conspicuous sheathing petioles, peculiar 

 odors derived from oils and resins, small flowers that are usually 

 white or yellow and grouped into flat-topped inflorescences (um- 

 bels), which are usually surrounded by bracts, called the involucre 

 (Fig. 324, A, B). These features are well seen in the wild carrot 



Fig. 324. A common form of the Umbellales : A, stems of wild carrot 

 (Daucus) with flowers arranged in compound umbels. B, inflorescence 

 with all the umbels removed but one — in, bracts of the involucre. C, 

 forms of flowers, the one on the right being a flower from the margin of 

 the umbel. The corolla is enlarged and irregular, thus adding to the con- 

 spicuousness of the inflorescence. D, corolla partially removed to show 

 the epigynous character of the flower and the cushion-like nectary at the 

 base of the styles ; 0, ovary. E, fruit splitting into two nutlets. F, portion 

 of hollow stem with sheathing leaf base. 



which has become a troublesome weed in pasture lands and mea- 

 dows. The calyx lobes are very small, a feature likely to be seen 

 in any epigynous flower. The incurved petals alternate with the 

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