314 THE CROSSING OF FLOWEBS. 



the pollen from fertilizing the stigmas of the same flower, i.e. for preventing self- 

 fertilization or autogamy. For example, the relative position of the anthers and 

 stigmas in the flowers of the Arrow-grass (Triglochin; see fig. 237, p. 149) renders 

 it almost impossible for the pollen to reach the stigmas in the same flower, but the 

 possibility would not be excluded were the anthers to shed their pollen at the 

 time when the stigmas were capable of being fertilized. Since, however, in the 

 flowers of the Arrow-grass, the stigmas are quite dried up at the the time of dehis- 

 cence, autogamy is quite impossible, and so far dichogamy is a completion of the 

 contrivances mentioned. But such cases of complete dichogamy as in the Arrow- 

 grass, the Wall Pellitory, and the Grass of Parnassus, &c., are comparatively rare, 

 and this explanation will not hold for the great bulk of hermaphrodite flowers 

 which are incompletely dichogamous. Still less will it apply to monoecious and 

 dioecious plants. Here there is no question of autogamy or self-fertilization, and 

 for this reason all hypotheses founded on the prevention of self-fertilization by 

 dichogamy are futile. 



We cannot suppose, however, since the non-simultaneous maturation of the 

 sexes is a phenomenon which occurs in most — ^perhaps in all — ^plants, that this 

 contrivance has no meaning. I will now endeavour to elucidate the significance of 

 dichogamy and invite the reader, first of all, to enter one of the Willow plantations 

 which have been briefly described above. The Purple Willow (Salix purpurea) is 

 just beginning to bloom. The pistillate flowers already display mature stigmas; 

 but the staminate flowers are still behind, and not a single anther has opened. The 

 staminate flowers of the Osier (Salix viminalis), on the other hand, growing in the 

 same clump with the Purple Willow, are in their prime. The pollen of the Osier is 

 to be had in any abundance. Numerous bees have been attracted by the scent and 

 colour of the male catkins, and they buzz from bush to bush, sucking the honey 

 and collecting pollen. They are not dainty in their work, and do not limit them- 

 selves to one species but fly impartially to the Purple Willow, to the Osier, or to other 

 species of Willow which may happen to be present. Now, if a bee comes to suck 

 the honey from the pistillate flowers of the Purple Willow, after it has just visited 

 another Willow bush, where it has covered itself with pollen, obviously that bush 

 must have been the Osier, Sweet Willow, Sallow Willow, or some other species, 

 whose staminate flowers have already developed so far as to render their pollen 

 accessible. It cannot have been a Purple Willow, because not a single anther of this 

 species in the whole neighbourhood has yet opened. But since the stigmas of the 

 Purple Willow are thus fertilized by the pollen of the Osier, &c., hybridization 

 occurs. Two or three days later, a legitimate crossing may take place, for, by this 

 time the anthers of the Purple Willow will have protruded from the staminate 

 flowers and opened widely, and abundance of pollen will be afforded to insects. 

 These are not slow to visit the now accessible flowers, and they remove some of the 

 poUen and transfer it to the stigmas of the same species which are still capable of 

 being fertilized. Thus at the commencement of flowering hybridization is alone 

 possible, and legitimate cross-fertilization cannot take place till some time later. 



