Chap. XVII. 



SELF-IMPOTENT PLANTS. 



135 



pollen-masses, after an interval of eleven days, conld not be distin- 

 guished except by the difference of their caudicles, which, of course, 

 undergo no change. Fritz Miiller has, moreover, made a large number 

 of crosses between orchids belonging to distinct species and genera, 

 and he finds that in all cases when the flowers are not fertilised their 

 footstalks first begin to wither; and the withering slowly spreads upwards 

 until the germens fall off, after an interval of one or two weeks, and in 

 one instance of between six and seven weeks ; but even in this latter case, 

 and in most other cases, the pollen and stigma remained in appearance 

 fresh. Occasionally, however, the pollen becomes brownish, generally on 

 the external surface, and not in contact with the stigma, as is invariably 

 the case when the plant's own pollen is applied. 



Fritz Miiller observed the poisonous action of the plant's own pollen 

 in the above-mentioned Oncidium flexuosum, 0. unicorne, pules (?), and in 

 two other unnamed species. Also in two species of Eodriguezia, in two of 

 Notylia, in one of Burlingtonia, and of a fourth genus in the same group. 

 In all these cases, except the last, it was proved that the flowers were, 

 as might have been expected, fertile with pollen from a distinct plant 

 of the same species. Numerous flowers of one species of Notylia were 

 fertilized with pollen from the same raceme; in two days' time they 

 all withered, the germens began to shrink, the pollen-masses became dark 

 brown, and not one pollen-grain emitted a tube. So that in this orchid 

 the injurious action of the plant's own pollen is more rapid than with 

 Oncidium flexuosum. Eight other flowers on the same raceme were 

 fertilized with pollen from a distinct plant of the same species : two of 

 these were dissected, and their stigmas were found to be penetrated by 

 numberless pollen-tubes ; and the germens of the other six flowers became 

 well developed. On a subsequent occasion many other flowers were 

 fertilized with their own pollen, and all fell off dead in a few days ; whilst 

 some flowers on the same raceme which had been left simply unfertilised 

 adhered and long remained fresh. We have seen that in cross-unions 

 between extremely distinct orchids the pollen long remains undecayed ; 

 but Notylia behaved in this respect differently ; for when its pollen was 

 placed on the stigma of Oncidium flexuosum, both the stigma and pollen 

 quickly became dark brown, in the same manner as if the plant's own 

 pollen had been applied. 



Fritz Miiller suggests that, as in all these cases the plant's own pollen is 

 not only impotent (thus effectually preventing self-fertilization), but like- 

 wise prevents, as was ascertained in the case of the Notylia and Oncidium 

 flexuosum, the action of subsequently applied pollen from a distinct 

 individual, it would be an advantage to the plant to have its own pollen 

 rendered more and more deleterious; for the germens would thus quickly 

 be killed, and, dropping off, there would be no further waste in nourishing 

 a part which ultimately could be of no avail. Fritz Muller's discovery 

 that a plant's own pollen and stigma in some cases act on each other as 

 if mutually poisonous, is certainly most remarkable. 



We now come to cases closely ' analogous with those just 



