500 



BOTANY 



PART II 



render the self-fertilisation of hermaphrodite flowers completely im- 

 130ssi])le. It is obvious that dioecism completely prevents self-fertilisa- 

 tion, and that mona?cism at least hinders the pollination of the flowers 

 with pollen from the same plant. A similar result is brought about 

 when the two kinds of sexual organs of a hermaphrodite flower mature 

 at different times. This very frequent condition is known as 



DICHOGAMY. 



There are obviously two possible cases of dichogamy. Eitlier the stamens mature 

 first and the pollen is shed before the stigmas of the same flower are receptive ; 



the plant is known as protandrous (proterandrous, 

 androgynous). On the other hand the style with 

 its stigma may ripen first, before the pollen is ready to 

 be shed ; the plant is PROTOGYNors (proterogjnious, 

 gynandrous). 



Protandry, the earlier maturing of the male 

 sexual organs, is the more frequent form of dicho- 

 gamy. It occurs in the flowers of the Geraniaceae, 

 Campanulaceae, Compositae, Lobeliaceae, Umbelli- 

 ferae, Malvaceae, etc. The anthers, in this case, 

 open and discharge their pollen at a time when the 

 stigmas of the same flowers are still imperfectly 

 developed and not ready for pollination. Accordingl3^ 



PROTAKDROrS FLOWERS CAN ONLY BE FERTILISED BY 

 THE POLLKN OF YOUNGER FLOWERS. 



In the less frequent protogyny the female sexual 

 organs are ready for fertilisation before the jjollen of 

 tlie same flowers is ripe, and the stigma is usually 

 pollinated and withered before the pollen is shed ; so 

 that the protogynous flowers must be fertilised 



BY THE POLLEN OF OLDER FLOWERS {Anthoxailthum 



odoratum, Luzula pilosa, ScrophuJaria, nodosa, Aristo- 



lochia clematitis, Rcllehorus, Magnolia, Plantago, 



Fig. 455). 



A still more complicated method of eff'ecting 



cross-fertilisation, because involving also morpho- 

 logical and anatomical difi'ereuces 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. In contrast 

 to the unlimited possibility of crossing in other hermaphrodite flowers, there 

 is here a limitation to certain classes of individuals of the species. A good 

 example of heterostyled flowers is afl'orded by the Chinese Primrose (Fig. 456). 

 This plant has two forms of flowers, long-styled {L) and short-styled (A'), while the 

 positions of the stigmas and anthers in the two kinds of flowers are exactly reversed. 

 The pollen grains of the short-styled flowers, moreover, are larger, and the stig- 

 matic papillae shorter, than in those with the longer styles (;;, P, and n, A'). The 

 purpose of such morphological and anatomical ditt'erences existing between flowers 

 of the same species was first understood after they were discovered by Darwin to 

 be a contrivance for cross-pollination. Fertilisation is most successful in such cases 

 Avhen the pollination of the stigmas is eifected by the pollen of anthers correspond- 

 ingly situated. By such a "legitimate" fertilisation, more and better seeds are 



Fig. 455. — Inflorescence of Plaii- 

 tago media with protogynous 

 flowers. The upper, still closed 

 flowers (?) have protruding 

 styles ; the lower (S) have 

 lost their styles, and disolcse 

 their elongated stamens. 



