790 LECTURE XLV. 



, ' No less remarkable is Hekrostylism, in connection with the mutual fertili- 

 sation of different plants of the same species with androgynous flowers. The 

 individuals of the same plant in this case differ as regards their sexual organs : 

 the one individual forms exclusively flowers with a long style (elevated stigma), 

 and short filaments (depressed anthers), the other individual on the contrary, 

 flowers with depressed stigma and elevated anthers ; we have in this case, then, 

 in the same species of plant individuals with macrostylous and others with 

 microstylous flowers. Examples are Linum perenne, Primula sinensis and other 

 Primulaceae. The case also occurs, however, as in many species of Oxalis and 

 in Lythrum Salicaria, that three degrees of length of the sexual organs are met 

 with in the flowers of three specimens of the same species ; in addition to those 

 with macrostylous and those with microstylous flowers, there is found also one 

 with mesostylous flowers. Now in these cases of heterostylism Darwin and 

 Hildebrand have demonstrated that fertilisation is only possible {Linum perenne), 

 Or at any rate only has the best result, when the pollen of the macrostylous 

 flowers is transferred to the microstylous stigma of another plant, and the pollen 

 of the microstylous flowers to the macrostylous stigma of another plant. Where 

 three lengths of the styles occur, fertilisation is most successful, according to the 

 same rule extended, when the pollen is transferred to that stigma which, in 

 another flower, stands at the same level as the anthers from which the pollen is 

 derived. 



While in the numerous diclinous and dichogamous Pha,nerogams, and in those 

 to be mentioned below, insects carry the pollen of one flower to the stigma of 

 another, it occurs but relatively seldom that the pollination of one flower by another 

 is also accomplished without the aid of insects ; e. g. in some Urticacese, such as 

 Ptlea, and Morese, such as Broussonetia, where the anthers flying suddenly from the 

 bud scatter their light pollen in the air as delicate clouds of dust, which is blown 

 to the female organs of other flowers. Still more simple is the case of the Rye ; 

 the flowers in the ears of the Rye open singly, mostly in the morning, and 

 the rapidly elongating filaments thrust the mature anthers out of the glumes; 

 the anthers then hang down on the long filaments, and at once open and let the 

 heavy pollen fall. It falls on to the stigmas of flowers standing lower down 

 on the same spike or on neighbouring spikes, the swinging of the haulms 

 in the wind promoting the process. Rimepau has shown, moreover, that the 

 Rye is 'self-sterile,' that the individual flower can fertilise neither itself nor the 

 different flowers of an ear, nor can the different ears of one and the same plant 

 pollinate one another with success, although no mechanical hindrance to this 

 exists. 



Considering the effort so clearly expressed among t% Cryptogams, and still 

 more among the Phanerogams, to avoid fertilisation within the same bi-sexual apparatus 

 (self-fertilisation) it is a very striking fact that among the Angiosperms several 

 plants occur which form two kinds of androgynous flowers, namely, large ones which 

 are generally accessible to fertilisation by the pollen of other flowers, and small, 

 more or less abortive and occasionally subterranean (cleistogamous) flowers, which 

 never open, and the pollen of which sends its tubes directly from the anthers to 

 the stigma and fertilises the ovules; we have here, therefore, on the same individual 



