122 GOOD FROM CROSSING. Chap. XVII 



and in other cases all those down one side of the spike, with pollen 

 of other varieties, and the remaining flowers with their own pollen. 

 I saw these plants when the capsules were nearly mature, and 

 their curious arrangement at once brought full conviction to the 

 mind that an immense advantage had been derived from crossing 

 these hybrids. 



Lastly, I have heard from Dr. E. Bornet, of Antibes. who has 

 made numerous experiments in crossing the species of Cistus, but 

 has not yet published the results, that, when any of these hybrids 

 are fertile, they may be said to be, in regard to function, dioecious ; 

 " for the flowers are always sterile when the pistil is fertilised by 

 " pollen taken from the same flower or from flowers on the same 

 " plant. But they are often fertile if pollen be employed from a 

 " distinct individual of the same hybrid nature, or from a hybrid 

 i: made by a reciprocal cross." 



Conclusion. — Tliat plants should be self-sterile, although 

 both sexual elements are in a fit state for reproduction, appears 

 at first sight opposed to all analogy. With respect to the 

 species, all the individuals of which are in this state, although 

 living under their natural conditions, we may conclude that 

 their self-sterility has been acquired for the sake of effectually 

 preventing self-fertilisation. The case is closely analogous 

 with that of dimorphic and trimorphic or heterostyled plants, 

 which can be fully fertilised only by plants belonging to a 

 different form, and not, as in the foregoing cases, indifferently 

 by any other individual of the species. Some of these hetero- 

 styled plants are completely sterile with pollen taken from 

 the same plant or from the same form. With respect to 

 species living under their natural conditions, of which only 

 certain individuals are self-sterile (as with Beseda lutea), it 

 is probable that these have been rendered self-sterile to ensure 

 occasional cross-fertilisation, whilst other individuals have 

 remained self-fertile to ensure the propagation of the species. 

 The case seems to be parallel with that of plants which 

 produce, as Hermann Miiller has discovered, two forms 

 — one bearing more conspicuous flowers with their structure 

 adapted for cross-fertilisation by insects, and the other form 

 with less conspicuous flowers adapted for self-fertilisation. 

 The self-sterility, however, of some of the foregoing plants 

 is incidental on the conditions to which they have been 

 subjected, as with the Eschscholtzia, the Verbascum phoe- 



