348 AUTOGAMY. 



saccate interiors. The ovary is spindle-shaped, and bears at the end of a short style 

 a stigma covered with little papill:i\ Tin- stamens, four in number, rest with their 

 dorsal surfaces in contact with the ovary: their anthers face outwards, are lanceolate 

 in shape, and each carries on its apex a small foliaceous scale like the point of a 

 spear (see fig. 298 4 ). The flowers are protogynous, or, in other words, the papillose 

 stigma, which is hemmed in by the four scales just referred to, is already mature at 

 a time when the anthers are still closed (tig. 298 l ). At this stage the stigma may 

 be dusted with pollen from other flowers. The next step is the dehiscence of the 

 bilocular anthers, which takes place in a very peculiar manner. The wall of each 

 anther-half which faces outwards becomes detached, and is lifted up in the form of 

 a flap with the whole of the pollen of the corresponding cavity sticking to its inner 

 surface. The two flaps then shrivel and roll up, with the originally internal surface 

 outside, until the only point of attachment is just below the pointed scale at the 

 apex of the anther ; the free flaps arch over this foliaceous anther tip (fig. 298 5 ), 

 and also over the stigma, which is close to the scale (fig. 298 -). The process is simul- 

 taneous in all four anthers, the result being that the stigma is roofed over bv a vault 

 composed of eight curved flaps, and as the exposed surfaces of the latter are thickly 

 coated with pollen, the whole arch presents externally a continuous covering of that 

 material (see fig. 298 2 ). Should insects now visit the flower to obtain the honey 

 stored in the slipper-shaped petals they must brush past this pollen-covered vault ; 

 nor can they fail to be besmeared at the very spot where at a previous stage in the 

 flower's development they would have had to brush against the stigma. This con- 

 dition of affairs lasts usually for two days. In the meantime changes affecting the 

 pistil are taking place which, notwithstanding their inconspicuous nature, are of the 

 greatest importance in relation to the eventual accomplishment of autogamy. If the 

 stigma is not dusted by insects with foreign pollen at the first expansion of the 

 flower the pistil now elongates sufficiently to raise the stigma into the vaulted cavity 

 just described ; and as the flaps composing the vault curl up still more (fig. 298 6 ) 

 as the flower approaches its end, the stigma must inevitably come into contact 

 with the pollen adherent to them (cf. fig. 298 3 ). 



A similar process is observed to take place in several Cruciferse, of which the 

 Charlock (Sinapsis arvensis) is a typical instance. The flowers of this plant are 

 protogynous. The bud opens early in the morning, revealing still closed anthers 

 with their faces turned inwards (introrse), whilst the stigma is already mature and 

 projecting somewhat above the anthers. At this stage of development no pollen 

 except what is brought by insects can be deposited on the stigma. A day later the 

 flower presents quite a different appearance. The four longer stamens have stretched 

 and curved a little away from the axis, and the anthers are lifted above the stigma. 

 By a rapidly executed twisting of the filaments the anthers have been turned round 

 so as to face outwards, and extrorse sutural dehiscence has meantime taken place. 

 The stigma is completely withdrawn from observation, and is also safe from any 

 possibility of being dusted with pollen, for the anthers in the same flower have 

 turned away their pollen-coated faces, nor is it possible for any extraneous pollen 



