336 AUTOGAMY. 



alone is possible, in the evening of the first day autogamy takes place, and on 

 the next day pollen is again supplied to fertilize younger flowers — an alternation 

 which clearly shows that autogamy is not invariably merely a last stage in the 

 phenomenon of flowering. 



Similar events occur in several small-flowered species of the Crane's Bill 

 (e.g. Geranium coluvibimim, G. lucidum, G. Robertianum). In the middle of 

 the newly-opened flower is a receptive stigma with five radiating arms, and around 

 it are ten stamens, all of which are still closed. Five of the stamens are longer 

 than the rest, and hold their anthers nearly on a level with the stigma; the 

 other five anthers form a belt underneath the stigma. By the evening of the 

 first day the anthers of the longer stamens are already open, and transfer their 

 pollen to the tips of the adjacent stigmatic lobes. In Geranium lucidum the 

 phenomenon is not even delayed till the evening, but takes place four hours after 

 the flowers open. The flowers are not, however, then over. They close for the 

 night, and nod or droop to protect the pollen (see figs. 225 ^ and 225", p. 121), but 

 next morning they again become erect. The five stamens standing in front of 

 the petals then grow until the anthers reach the niches between the radiating 

 lobes of the stigma, whereupon there is a transference of pollen to these lobes. 

 Some of the anthers are afterwards lifted still higher, evidently for the purpose 

 of dispersing, by aid of insects, such portion of the pollen as has not been applied 

 to the process of autogamy. 



Several Convolvulacete, of which the well-known Ipomcea purpurea is a type, 

 have only two or three of their five stamens adapted to autogamy. The stamens, 

 which are parallel to the style and usually adherent to it, are of unequal length, 

 the shortest being 9 mm., the longest 17 mm., and the others 11 mm., 13 mm., 

 and 15 mm. in length respectively. The anthers consequently stand at diflerent 

 heights and at the same time they are so disposed relatively as not to cover one 

 another, an arrangement which has the advantage of presenting a comparatively 

 large expanse of pollen along the passage leading to the honey in the interior of 

 the flower. But even the anther of the longest stamen is 3 mm. lower than the 

 stigma when the flower first opens. Owing to this arrangement and to the cir- 

 cumstance that the flowers are protogynous, only cross-fertilization through the 

 intervention of insects can take place at the commencement of their flowering 

 period. Later on, however, there is a lengthening of the stamens and the anthers 

 pertaining to the longest two or three reach the same level as the stigma, and 

 yield up their pollen to it. The process of autogamy is further facilitated by the 

 involution of the corolla, which occurs at the close of flowering, whereby the 

 anthers coated with pollen are pressed against the stigma. 



From these Convolvulacese we pass to a long series of protandrous Caryophyl- •; ^ 

 lacese, mostly annual plants, such as Agrostemma Githago, Saponaria Vaccaria, '\ 

 and Silene conica, in which the anthers are brought into contact with the stigmas ^ 

 by a similar elongation of the stamens. The various changes occurring in flowers / 

 of the kind ensue with great regularity as follows:— (1) The petals separate, leaving 



