286 



BOTANY 



stigmas of the same flowers ar 



Fig. 217.— Inflorescence of Plan- 

 tago media with protogynous 

 flowers. The upper, still closed 

 flowers (?) have protruding 

 styles; the lower (<$) have 

 lost their styles, and disclose 

 their elongated stamens. 



discovered by DarwIn to be 

 most successful in such cases 



e still imperfectly developed and not ready for pollina- 

 tion. Accordingly, peotandrous flowers can only 



BE FERTILISED BY THE POLLEN OF YOUNGER FLOWERS. 



In the less frequent Protogyny the female sexual 

 organs are susceptible to fertilisation before the pollen 

 of the same flowers is ripe ; so that the protogynous 

 FLOWERS MUST BE FERTILISED BY THE POLLEN OF 

 older flowers (Anthoxanthum odoratum, Luzula 

 pilosa, ScropMdaria nodosa, Helleborus, Magnolia, 

 Plantago media, Fig. 217). 



A still more complicated method of effecting 

 cross -fertilisation, because involving also morpho- 

 logical and anatomical differences of structure, results 

 from Heterostyly, or the peculiarity of some species 

 of plants of producing stigmas and anthers which 

 vary in height in different individuals of the same 

 species. A good example of heterostyled flowers is 

 afforded by the Chinese Primrose (Fig. 218). Tliis 

 plant has two forms of flowers, long-styled (£) and 

 short-styled (K), while the positions of the stigmas 

 and anthers in the two kinds of nowers are exactly 

 reversed. The pollen grains of the short-styled 

 flowers, moreover, are larger, and the stigmatic papillee 

 shorter, than in those with the longer styles (p, P, 

 and n, A 7 ). The purpose of such morphological and 

 anatomical differences existing between Sowers of the 

 same species was first understood after they were 

 a contrivance for cross-pollination. Fertilisation is 

 when the pollination of the stigmas is effected by the 



Fig. 218. — Primula sinensis ; two heterostyled nowers from different plants. L, Long-styled ; K, 

 short-styled flowers; G, style; S, anthers; P, pollen - grains ; and N, stigmatic papillse of 

 the long-styled form ; p and n, pollen-grains and stigmatic papilla; of the short-styled form. 

 (P, N,p,,n, xllO.) 



pollen of anthers correspondingly situated. By such a "legitimate" fertilisation 

 more and better seeds are produced than by "illegitimate" fertilisation, and in 



