DICHOGAMF, HETEROSTYLISM, HERKOGAMF. 789 



distributions of sexes in the Cryptogams, that it is most conspicuously shown that 

 the co-operation of the sexual cells of near relationship must be injurious for the 

 existence of most plants, since such various and often perfectly astounding means 

 are employed for avoiding fertilisation within a hermaphrodite sexual apparatus. 



One of the commonest and simplest means is Dichogamy — i. e. the non- 

 simultaneous development of the two sexual organs in one and the same andro- 

 gynous sexual apparatus, so that the closely related sexual cells produced side 

 by side, compelled to exercise their functions at different times, and thus cannot 

 act together. The male cells must therefore unite with the female ones of 

 another androgynous sexual apparatus. This is very generally the case with 

 the flowers of Angiosperms, as well as with most Fern-prothallia and those 

 Characeas which are not dioecious, where, although the oogonium arises close 

 beside the antheridium, it attains sexual maturity later than the latter (very 

 conspicuous for example in Nitella flexilis). 



In the case of the dichogamous flowers of Phanerogams, insects are employed 

 for the transference of the pollen on to the stigma of other flowers, and for this 

 purpose special mechanisms exist in the floral organs : we shall examine these 

 later on. In the dichogamous Nitellas and Fern-prothallia the movement of the 

 antherozoids suffices, the close growth of the plants promoting their access to 

 archegonia of neighbouring prothallia, or to the oogonia on other leaves of the 

 Nitella,- or even of other plants. Whether dichogamy exists in the case of the 

 Algae above mentioned, and some Muscinese, is questionable, but at any rate the 

 possibility is given, by means of the motility of the antherozoids and other 

 conditions which here prevail, that they come in contact with the oospheres of 

 other plants or of other branches of the, same plant. 



In the Angiosperms however, in addition to the frequent dichogamy, quite 

 other arrangements also occur which have exclusively the object of accomplishing 

 the transference of the pOllen of hermaphrodite plants with the aid of insects to 

 the stigma of other flowers, or even of the flowers of other plants. In most 

 Orchidese, Asclepiadeae, Viola, ^Sic, the sexual organs of each individual flbvEcr 

 are developed simultaneously, it is true, but at the time of sexual maturity there' 

 are mechanical arrangements which prevent the pollen coming on to the stigma 

 of the same flower {Herkogamy) : it must be transferred to other flowers by 

 insects. 



In other cases, as in Corydalis ,cava (pointed out by Hildebrand) the pollen 

 actually falls on the stigma of the same flower, but it is here without effect, and 

 it is only effectual in fertilisation when it is transferred to the stigma of another 

 flower, and only completely so if it is carried to the flowers of another plant of 

 the same species. T||fs plant, therefore, is only morphologically androgynous : 

 physiologically it is dioecious. The Orchid Oncidium microchilum behaves 

 similarly, according to John Scott, in so far as the pollen placed on the stigma 

 of the same flower does not fertilise it, while it is able to fertilise another indivi- 

 dual, and also that the female organ is fertilised by foreign pollen. Pollen and 

 stigma of the same flower are thus functionally capable, but only for the organs 

 of a foreign flower. Similar relations were observed by Gartner in Lobelia 

 fulgens and Verbascum nigrum, and by Fritz Miiller in Bignonias. 



