252 CONCLUDING REMARKS CHAP. VL 



that the grains of the two forms should differ in size 

 relatively to the length of the pistil which the tubes 

 have to penetrate, but that in the latter case it would 

 not be necessary that the grains should thus differ. 

 Whether this explanation can be considered satisfac- 

 tory must remain at present doubtful. 



There is another remarkable difference between the 

 forms of several heterostyled species, namely in the 

 anthers of the short-styled flowers, which contain the 

 larger pollen-grains, being longer than those of the 

 long-styled flowers. This is the case with Hottonia 

 palustris in the ratio of 100 to 83. With Limnan- 

 tliemum Indicum the ratio is as 100 to 70. With the 

 allied Menyanthes the anthers of the short-styled form 

 are a little and with Villarsia conspicuously larger 

 than those of the long-styled. With Pulmonaria 

 anyustifolia they vary much in size, but from an 

 average of seven measurements of each kind the ratio 

 is as 100 to 91. In six genera of the Rubiacese there 

 is a similar difference, either slightly or well marked. 

 Lastly, in the trimorphic Pontederia the ratio is 100 

 to 88 ; the anthers from the longest stamens in the 

 short-styled form being compared with those from the 

 shortest stamens in the long-styled form. On the 

 other hand, there is a similar and well-marked differ- 

 ence in the length of the stamens in the two forms 

 of Forsythia suspensa and of Linum flavum ; but in 

 these two cases the anthers of the short-styled flowers 

 are shorter than those of the long-styled. The rela- 

 tive size of the anthers was not particularly attended 

 to in the two forms of the other heterostyled plants, 

 but I believe that they are generally equal, as is 

 certainly the case with those of the common primrose 

 and cowslip. 



The pistil differs in length in the two forms of every 



