SECT, ii 



PHYSIOLOGY 



311 



differences existing between flowers of the same species was first understood after 

 they were discovered by DARWIN to be a contrivance for cross-pollination. Ferti- 

 lisation is most successful in such cases when the pollination of the stigmas is 

 effected by the pollen of anthers correspondingly situated. By such a " legitimate " 

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

 tion, and in some cases (Linum perenne, Fagopyrum csculentum) legitimate 

 fertilisation alone is productive. Legitimate fertilisation is rendered more certain 

 by the fact that insects in visiting the flowers touch correspondingly placed sexual 

 organs with the same portions of their body. The flowers of Primroses have styles 

 of two different lengths (DIMORPHIC HETEROSTYLY) ; the same peculiarity is 

 exhibited by Piilmonaria, Hottonia, Fagopyrum, Linum. There are also flowers 

 with TRIMORPHIC HETEROSTYLY (Lythrum Salicaria, and some species of Oxalis), 

 in which there are two circles of stamens and three variations in the height of the 

 stigmas and anthers. 



\ 



Fio. 242. Primula sineiisis: two heterostyled flowersj'from- different plants. L, Long-styled; 

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

 the 'long-styled form; p and n, pollen-grains and- stij-'inatic papillae of the short-styled form. 

 (P, N, p, n, x lio.) 



The different forms of heterostylic plants do not only differ in the length of the 

 style and stamens, and in the colour and size, of the pollen grains, but according 

 to ERRERA commonly exhibit differences in form of leaf, in the size and colouring of 

 the flowers, and in the weight of the seeds. 



In a great number of flowers self-pollination is made mechanically impossible, 

 as their own pollen is prevented by the respective positions of the sexual organs 

 from coming in contact with the stigma (HERCOGAMY). In the Iris, for example, 

 the anthers are sheltered under the branched petaloid style, upon whose lip-like 

 stigma no pollen can come, unless through the agency of insects, and in the 

 protogynous flower of Aristolochin dcrnatitis pollination from the anthers, 

 which occupy a lower position on the column, is prevented. The conveyance of 

 pollen from the older to the younger flowers is effected in Aristolochia by small 

 insects. The flowers at first stand upright with a widely .opened mouth (Fig. 243 /) 

 and in this condition the insects can easily push past the downwardly directed 

 hairs clothing the tubular portion of the corolla and reachjthe dilated portion below. 

 Their exit is, however, pi-evented by the hairs until the stigma has withered and 

 the anthers have shed their pollen. When this has taken place (Fig. 243 IT) the 

 hairs dry up and the insects covered with pollen can make their way out and 



