G. M. Thomson. — On the Fertilization of Flowering Plants. 243 



In whatever manner flowers may be fertilized, it is now known that 

 pollen from a flower on a different plant seems to produce more and larger 

 seeds, from which spring finer and stronger plants, than result from fertili- 

 zation by pollen of the same flower applied to its own pistil. Hence 

 probably, to a certain extent at least, the advantages of, and tendency to, 

 separation of the sexual organs, which is so common a phenomenon among 

 New Zealand flowers. 



A point worthy of notice among entomophilous flowers is this, — that not 

 only have flowers become modified for fertilization by insects, but even by 

 certain insects only. Thus some are suited for fertilization by Lepi- 

 doptera alone, others by Diptera, Hymenoptera, or Coleoptera only, while 

 some are actually dependent on certain species of insects. This seems to 

 be the case with several species of the long-nectaried orchids of the sub- 

 tropical genus AngrcEcum, and with TrifoUum j^ratense (the common red 

 clover), which is apparently only fertilized by long-trunked bees of the 

 genus Boinbus. But even the converse probably holds good, viz., that many 

 insects have become modified in certain respects by their becoming to some 

 extent dependent on certain species of flowers. Thus it can only be moths 

 or butterflies with an extremely long proboscis which can obtain honey 

 from the whip -like nectary of AngrcBcum sesquipedale, which attains a length 

 of eleven inches (though no species has yet been found which accomplishes 

 this) ; and we can understand how completely dependent the flowers of this 

 plant are upon such moths, and how the insects themselves must be advan- 

 taged in that no others can compete with them for this supply of food. 



Among hermaphrodite flowers in which no special arrangement or con- 

 trivance exists for preventing self-fertilization, it does not follow that the 

 pistils are always pollinated by the stamens which are included in the same 

 perianth with them. Darwin* has pointed out that in very many plants 

 the flowers of which are quite fertile with their own pollen, the pollen from 

 other flowers is found to have a greater fertihzing power, and to produce 

 fertilization of the ovules even after their own pollea has been scattered on 

 the stigmas ; so that the visits of pollen- carrying insects to such flowers 

 are sure to be advantageous, even although not absolutely necessary for the 

 production of seed. Hermaphrodite flowers show every gradation between 

 perfect self- fertility, such as prevails in various species of Cruciferous and 

 Caryophyllaceous plants, and absolute self-sterility, as in Oxalis mageilanica, 

 in which no seed is produced even after the stigma has been abundantly 

 smeared with pollen from the adjacent anthers. This latter state of self- 

 sterility is, however, attained in a variety of ways. Thus in some flowers, 

 the anthers dehisce and scatter all their pollen before the stigmas are ready 



* " Cross- and Self-Fertilization of Plants," p. 391, 



