3 22 



CHAPTERS ON EVOLUTION. 



procuring the fertilisation of the long-styled flowers by the pollen of the 

 short- styled flowers, and vice versa. Such an interchange of pollen is 

 accomplished in a manner readily understood. Suppose that an insect 

 such as a humblebee first visits a "long-styled" flower (Fig. 226, 2), 

 drawn to the primrose in search of the nectar which this flower, the 

 cowslip, and other members of the genus secrete in quantity. The 

 proboscis of the bee will be thrust into the tube of the corolla, and 

 in the act of nectar-gathering it will unconsciously dust its proboscis 

 with pollen nearer the tip of that organ than the base. Suppose 

 next that the bee, with its pollen-laden proboscis, flies to another 

 primrose plant of the " short-styled " variety. The proboscis, inserted 

 therein as before, will corne into contact with the low-lying stigma, 

 and upon this surface will be left the pollen which was gathered from 

 the stamens of the long-styled flower. Thus interchange the first is 

 accomplished. But when visiting the "short-styled " flower (i), the 

 bee's proboscis, coming in contact with the stamens (placed at the top 

 of the flower), is dusted near the base with short-styled pollen. Hence 

 the next visit paid to a " long-styled " flower results in the placing of 

 pollen from' the short-styled flower upon the stigma of the long-styled 

 primrose. The stigma of the latter is placed, as we have seen, at the 



top of the flower (Fig. 226, 2), and it 

 is thus well calculated to meet the 

 base of the bee's " tongue," which has 

 been dusted with short-styled pollen. 

 Interchange the second is thus ac- 

 complished, and the cross -fertilisation 

 of. the primrose race becomes a 

 matter of well-nigh absolute certainty. 

 As indicated in the annexed figure, 

 from Mr. Darwin (Fig. 226), the "le- 

 gitimate " (A) fertilisation is that which 

 occurs when pollen from the long- 

 styled form is placed on the short- 

 styled pistil, and vice versa. The 

 " illegitimate " fertilisation (B) is self- 

 fertilisation in either case; namely, through the pollen of either 

 flower being placed upon its own stigma. Mr. Darwin's own words, 

 applying to his observations on the cowslip (Primula veris\ may 

 be read with interest as applying likewise to the similar arrange- 

 ment in the allied primrose. After noting that humblebees, and 

 likewise moths, visit these flowers, Mr. Darwin says : " It follows from 

 the position of the organs (anthers and stigmas) that if the 

 proboscis of a dead humblebee, or a thick bristle or rough 

 needle, be pushed down the corolla, first of one form and 

 then of the other, as an insect would do in visiting the two forms 



FIG. 226. 

 FERTILISATION OF PRIMROSE (Darwin). 



