THE FERTILISATION OF FLOWERS. 321 



transitional states between the two forms growing in a state of 

 nature. The cowslips and other allies of the primrose exhibit a like 

 disposition of parts. Thus, when we slit one of the primroses 

 longwise, we see that the stamens (Fig. 208 B, a) are placed high up on 

 the petals or near the top of the corolla, and the style is comparatively 

 short. In the other variety (Fig. 208 A, a), the stamens are placed far 

 down within the tube of the corolla, whilst the style is so long that 

 the stigma (sf) appears to block up the entrance to that tube, and 

 reaches to the top of the petals. Thus we speak of " short-styled " (B) 

 and " long-styled " (A) flowers in primrose and in all other plants in 

 which these conditions occur; whilst, popularly, the short-styled 

 forms are called " thrum- eyed," and the long-styled ones "pin-eyed." 

 Such a disposition of stamens and pistil also occurs in Pulmonaria 

 offirinalis, in Linum perenne, and in other plants, which are hence 

 called dimorphic, i.e. having two forms of flower. And in some 

 plants (e.g. Oxalis and the Spiked Loosestrife or Lythrum Salicaria), 

 three varieties of flowers are known, and these latter are named 

 trimorphic in consequence. 



Returning to our primroses, we find that the pollen-grains of the two 

 forms of flower differ in size. Those of the long-styled primroses (A) 

 are smaller than those of the short-styled flowers. Mr. Darwin remarks 

 of the pollen-grains of the latter flowers, that " before they were soaked 

 in water, they were decidedly broader, in proportion to their length, 

 than those from the long-styled ; after being soaked, they were rela- 

 tively to those from the long-styled as 100 to 71 in diameter, and more 

 transparent." Mr. Darwin also compared these two forms of flowers 

 in other respects. He found that the seeds of the short-styled 

 flowers " weighed exactly twice as much as those from an equal 

 number of long-styled plants/' the short-styled being the more pro- 

 ductive of the two forms. As final facts concerning the differences 

 between the two varieties, it may be noted that the stigma or head of 

 the pistil in the long-styled form is more distinctly globular and 

 roughened on its surface than that of the short-styled primroses-; 

 whilst the stigma in each form stands nearly, but not quite, on a level 

 with the anthers of the opposite variety. 



What can be affirmed, as a matter of observation, to be the mean- 

 ing and purpose of this diverse arrangement of stamens and pistils in 

 these plants ? Meaning it must have, and that one which is closely 

 bound up with the history of the species. So indeed, it was found, 

 when, through Mr. Darwin's researches, contributed to the Linnaean 

 Society's Transactions in 1862, it was clearly demonstrated that the 

 arrangement in question had. reference to the cross-fertilisation of the 

 primroses and of all other plants in which a like diversity of structure 

 was found. Mr. Darwin then pointed out that the structure of the 

 primrose was eminently adapted to favour the visits of insects as aids in 



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