424 



NA TURE 



\Sept. 24. 1874 



pitcher-like vessels, wliich secrete a sweetish liquid, and thus 

 attract numerous insects. These again bring birds, which can 

 hardly fail to brush against the flowers, and thus convey the 

 pollen from one to the other. 



In the flowers hitherto described, while the several species 

 offer the most diverse arrangements, we have met with no diffe- 

 rences within the limits of the same species, excepting those 

 dependent upon sex. I must now call attention to some cases in 

 which the same species possesses flowers of two or more kinds, 

 which sometimes, as in the Violet, are adapted to different con- 

 ditions ; but more frequently are so constituted as to ensure 

 cross-fertilisation. 



In some of the violets ( V. odorata, canina, &c.), besides the 

 blue flowers with which we are all so familiar, but which produce 

 very little seed, there are other autumnal flowers, almost without 

 petals and stamens, and which indeed have none of the appear- 

 ance of true flowers, but in which the seeds are produced. As 

 these curious flowers, however, have no relation to our present 

 subject, I shall not now dwell on thtm. 



I pass on to the genus Primula, which offers a most interesting 

 case of dimorphism. The Cowslip and Primrose resemble one 

 another in many respects, though the honey they secrete must be 



very different, for vpliile the Cowslip is habitually visited during 

 the day by humble bees, this is not the ease with the Primrose, 

 which, in Mr. Darwin's opinion, is fertilised almost exclusively 

 by moths. (Jour. Linn. Soc, vol. x. p. 438.) This, however, 

 is a digression. 



Corresponding differences occur in the Polyanthus and Auricula, 

 and had long been known to gardeners, and even to school 

 children (by whom the two kinds of flowers are known as " pin- 

 eyed " and " thumb-eyed "), but it was reserved for the genius 

 and perseverance of Mr. Darwin, to explain * the significance of 

 this curious phenomenon, and the important part it plays in the 

 economy of the flovi'er. Now that Mr. Darwin has pointed this 

 out it is sufficiently obvious : an insect thrusting its proboscis 

 down a primrose of the long-styled fonii would dust its proboscis 

 apart, which, when it visited a short-styled flower would come 

 just opposite the head of the pistil, and could not fail to deposit 

 some of the pollen on the stigma. Conversely an insect visiting 

 a short-styled plant would dust its proboscis at a part further 

 from the tip, and which, when it subsequently visited a long- 

 styled flower, would again come just opposite to the head of the 

 pistil. II Ci.cc we .see that by this beautiful arran!;ement insects 

 will C:irry the pollen of the long-styled form to tlie short-styled, 

 and vice vci-sd. 



There are other points in which ^the two forms differ from one 

 * Li'niienii yoiinml, 1862, p. 77. 



another : for instance, the stigma of the long-styled form is 

 globular and rough, while that of the short-styled is smoother, 

 and somewhat depressed. The pollen of the two forms is also 

 dissimilar, that of the long-styled being considerably smaller 



than the other, 7-7ooolhs of an inch in diame'er against ISHll 

 or nearly in the proportion of three to two ; a difference the im- 



li 1"' pi 



Fig. 25. Fig. =5. 



portance of which is obvious, for each has to give rise to a tube 

 which penetrates the whole length of the style, from the stigma 

 to the base of the flower, and the tube in the long-styled form 

 must therefore be nearly twice as long as in the other. Mr. Dar- 

 win has shown that much more seed is set if pollen from the one 

 form is placed on the pistil of the other, than if the flower is 

 fertilised by pollen of the same form, even if taken from a dif- 



■/ 4\ 



Fig 27. Fig. 28. Fig. 29. 



ferent plant. Nay, what is most remarkable, such unions in 

 Primula are more sterile than crosses between distmct, though 

 nearly allied species of plants, have in some cases been found 

 to be. 



The majority of species of the genus Primula appear to be 

 dimorphic, but not all.* 



Mr. Darwin has pointed out t that several species of Linium 



Fig. 30, 



Fig. 31. 



are dimorphic in the same manner as the Cowslip and Prim- 

 rose. I.ythntDt saliiirn'n^ however, + is even more remarkable, 

 since as was remarked by Vaucher, but first explained by Mr. 

 Darwin, it presents us with three distinct forms (each contain- 



* Scott, Proc. Linn. Soc, vol. viii., 1864, p. io, 

 t Jour. Linn. Soc, 1S63. p. 69. 

 I LiiuL Jour. 18O4, p. jfy. 



