THE ANGIOSPERMAE 



1289 



or female stage at the time when the flowers in the middle of the inflores- 

 cence are shedding pollen, while at the bottom the ovules may be already- 

 fertilized. Cross-poUination is favoured by this protogyny, but there is a 



Fig. 1220. — Plantago lanceolata. A, Spike with flowers in the stigmatic, staminate 

 and post-polHnation states. B, Flower in stigmatic state, anthers immature. C, 

 Flower in open, staminate state, vertical section. D, Flower with mature 

 stamens. E, Single stamen. (C and D, after Le Maoiit ami Decaisne.) (See 

 also Fig. 12 1 2.) 



brief period of overlap, when pollen may be blown on to the stigma of the 

 same flower. Such a flower, however, will have had an opportunity to be 

 cross-pollinated and self-pollination is only a secondary possibility. 



4. Explosive Type 



The discharge of pollen by explosion of the anthers is known in a number 

 of insect-pollinated flowers, but it is rare among wind-pollinated ones. In- 

 deed the mechanism is known chiefly from the members of the Urticaceae 

 and we may cite Urtica dioica as an example (Fig. 1221). 



As a rule the male and female flowers are borne on separate plants, 

 though cases are not uncommon in which both sexes occur on the same 

 plant. In this case there are usually female inflorescences at the top, mixed 

 inflorescences in the middle and male ones towards the bottom of the plant. 

 The female flowers consist of a four-parted perianth enclosing a single 

 ovary which bears a brush-like stigma and contains one ovule. The male 

 flower also consists of four perianth segments, opposite each of which a 

 stamen develops. Each stamen is bent inwards in such a way that the 



