114 GOOD FKOM CROSSING. Chap. XYII. 



had been possible, for the anthers not only touch the stigma, but 

 the pollen-tubes were seen by Dr. Hilclebrand to penetrate it; 

 nevertheless these eighty-four flowers did not produce a single 

 seed-capsule ! This whole case is highly instructive, as it shows 

 how widely different the action of the same pollen is, according as 

 it is placed on the stigma of the same flower, or on that of another 

 flower on the same raceme, or on that of a distinct plant. 



With exotic Orchids several analogous cases have been observed, 

 chiefly by Mr. John Scott. 69 Oncidium sphacelation has effective 

 pollen, for Mr. Scott fertilised two distinct species with it: the 

 ovules are likewise capable of impregnation, for they were readily 

 fertilised by the pollen of 0. divaricatum ; nevertheless, between 

 one and two hundred flowers fertilised by their own pollen did not 

 produce a single capsule, though the stigmas were penetrated by 

 the pollen-tubes. Mr. Eobertson Munro, of the Royal Botanic 

 Gardens of Edinburgh, also informs me (1864) that a hundred and 

 twenty flowers of this same species were fertilised by him with 

 their own pollen, and did not produce a capsule, but eight flowers, 

 fertilised by the pollen of 0. divaricatum, produced four fine cap- 

 sules : again, between two and three hundred flowers of 0. divari- 

 catum, fertilised by their own pollen, did not set a capsule, but 

 twelve flowers fertilised by O.flexuosum produced eight fine cap- 

 sules : so that here we have three utterly self-impotent species, with 

 their male and female organs perfect, as shown by their mutual 

 fertilisation. In these cases fertilisation was effected only by the 

 aid of a distinct species. But, as we shall presently see, distinct 

 plants, raised from seed, of Oncidium flexuosum, and probably of the 

 other species, would have been perfectly capable of fertilising each 

 other, for this is the natural process. Again, Mr. Scott found that the 

 pollen of a plant of (J. microchilum was effective, for with it he ferti- 

 lised two distinct species ; he found its ovules good, for they could 

 be fertilised by the pollen of one of these species, and by the pollen 

 of a distinct plant of 0. microchilum ; but they could not be ferti- 

 lised by pollen of the same plant, though the pollen-tubes penetrated 

 the stigma. An analogous case has been recorded by M. Riviere, 70 

 with two plants of 0. cavendishianum, which were both self-sterile, 

 but reciprocally fertilised each other. All these cases refer to the 

 genus Oncidium, but Mr. Scott found that Maxillaria atro-nibem 

 was "totally insusceptible of fertilisation with its own pollen," but 

 fertilised, and was fertilised by, a widely distinct species, viz. M. 

 sjuahns. 



As these orchids had been grown under unnatural conditions in 

 hot-houses, I concluded that their self-sterility was due to this 

 cause. But Fritz Miiller informs me that at Desterro, in Brazil, he 



09 « Proc. Bot. Soc. of Edinburgh,' Linn. Soc.,' vol. viii. Bot., 1864, p 



May, IS jo : these observations are 162. 



given in abstract, and others are :o Prof. Lecoq, 'De la FJcondation, 



idded, in the ' Journ.d of Proc. of 2nd edit.. 1862, p. 7'3. 



