460 



SPERMATOPHYTES (SEED PLANTS) 



characteristic of Angiosperms. The stamens are microsporo- 

 phylls and the pistils are megasporophylls. A typical flower is, 

 therefore, essentially an_ association of sporophylls surrounded 

 by a perianth, and, in so far as a flower is an association of sporo- 

 phylls, it does not differ 

 fundamentally from a stro- 

 bilus. In passing from the 

 simplest Angiosperms, 

 where there are flowers 

 that have no perianth, to 

 those Angiosperms having 

 typical flowers, all grada- 

 tions between a typical 

 strobilis and a typical 

 flower can be found. It is, 

 therefore, impossible to de- 

 fine a flower so as to in- 

 clude the flowers of all 

 Angiosperms and at the 

 same time separate the 

 flower from the strobilus. 

 The flowers of Angiosperms 

 and the strobili of the Gym- 

 nosperms and P t e r i d o- 

 phytes differ in the char- 

 acter of their sporophylls 

 more than in any other 

 feature. 



Perianth. — The peri- 

 anth, usually consisting of 

 both sepals and petals, not 

 only protects the sporo- 

 phylls during their develop- 

 ment but also serves in 

 pollination, which in Angiosperms is done largely by insects. 

 At the base of the perianth occur nectar glands, which are further 

 adaptations to insect pollination. The perianth seems to have 

 arisen in two ways. In some cases there is evidence that the 

 parts of the perianth are modified sporophylls, while in other 

 cases they are apparently modified foliage leaves. 



Fig. 407. — The floral structures of a 

 typical flower. The floral structures com- 

 prise a perianth (a) composed of calyx 

 and corolla, a number of microsporophylls 

 or stamens each consisting of anther (c) 

 and filament (c), and a pistil (b) composed 

 of one or more megasporophylls with the 

 megasporangia or ovules (d) enclosed in an 

 ovary. 



