46 



INTRODUCTION 



mentioning these examples, goes on to say ('Nat. Hist. PI.,' Eng. Ed. i, II, p. 396): 

 ' It may be taken to be a general rule that the flowers adapted to cross-fertilization in 

 which no autogamy takes place are larger than those in which the accomplishment of 

 autogamy is assured. This phenomenon has been explained by the circumstance that 

 flowers destined to be crossed with others require to be more plentifully equipped 

 with the means of attracting insects than those which are certain to undergo pollina- 

 tion even if no insects visit them.' 



Fig. 4. Lythrum salicaria, L. (after Herm. Muller). (i) Long-styled flower. Seen from above, 

 after removal of the upper third of the calyx, corolla, and stamens. (2) Flower with style of medium 

 length similarly treated. (3) Flower with short style similarly treated a. Long stamens and style. 

 b. Medium stamens and style, c. Short stamens and style. (4) Flower with medium style seen obliquely 

 from the right front. 



Trimorphous flowers diff'er from one another in similar ways to the dimorphous, 

 as regards size of stigmatic papillae, pollen-grains, and the like. In the typical trimor- 

 phic plant Lythrum Salicaria, the pollen-grains of the long stamens are the largest, those 



of the medium stamens are of inter- 

 mediate size, and those of the short 

 stamens the smallest ; the anthers of 

 the long stamens are green, those 

 of the medium, and short are yellow ; 

 the filaments of the long stamens 

 are red, those of the medium and 

 short are green; the stigmatic pa- 

 pillae of the long-styled flowers are 

 noticeably longer than those of the 

 medium, and these are a little longer 

 than those of the short-styled; the 

 seeds of long-styled flowers are 

 larger than those of medium-styled 

 flowers, and these again are larger 

 than those of short-styled flowers. 



Epigaea, according to Asa Gray, 

 has even tetramorphous flowers, 

 differing from one another partly 

 in the length of the style, partly 

 in regard to the stigma and anthers. 



Fig. 5. Dtagrain of the possible legitimate and illegt- 

 ti-mate unions in a dimorphous plant (Primula) (after 

 Darwin). The arrows indicate the anthers from which pollen 

 must be brought to the stigma of either of the two forms, so 

 as to give a legitimate union (the straight horizontal lines), 

 or an illegitimate union (the curved lines on right and left). 

 (Loew.) 



