HVBRIDISA TION. 9 1 5 



two species belonging to different genera, the resulting hybrid may be termed a 

 variety-hybrid, species-hybrid, or genus-hybrid. 



Among Cryptogams only a few instances of hybridisation are known with 

 certainty. Thuret (Ann. des sci. nat., 1855) obtained hybrid plants by bringing 

 antherozoids of Fucus serratus into contact with oospheres of F. vesiculosus. In 

 some other families of Cryptogams forms have been found which have been sup- 

 posed, from their characters, to have a hybrid origin. Thus A. Braun (Verjiingung, 

 p. 329) adduces instances of hybrids between Mosses ^ PhyscGmitrium pyriforme and 

 Funaria hygrometrica, and between Physcomitrium fascicular e and Funaria hygro- 

 metrica, and between the following species of Ferns — Gymnogramme chrysophylla 

 and G. calomelana, G. chrysophylla and G. distans, and Aspidium Filix-mas and 

 A . spinulosum ^. 



The most important observations from a scientific point of view, which have 

 given us the clearest insight into the nature of the difference of sex, are however 

 those made on hybrids between flowering plants, resulting from the artificial convey- 

 ance of pollen from one species to another. Nageli has collected the results of 

 many thousand experiments on hybridisation made by Kolreuter in the last century, 

 and more recently by Knight, Gartner, Herbert, Wichura, and other observers. The 

 following facts are taken chiefly from Nageli's resume^. 



I. Only those forms which are closely related genetically can produce hybrids. 

 They are formed most easily between different varieties of the same species ; with 

 greater difficulty — but are still possible in a great number of cases — between two 

 species of the same genus ; of hybrids between species which belong to different 

 genera only a very few instances are known, and it is probable that in these cases 

 the species ought to be included in the same genus. The facility with which 

 hybrids can be produced varies extremely in different orders, families, and genera of 

 Angtosperms. The phenomenon is frequent among Liliacese, Irideae, Nyctaginese, 

 Lobeliacese, Solanaceae, Scrophulariacese, Gesneraceae, Primulaceae, Ericaceae, Ranun- 

 culaceae, Passifloraceae, Cactaceae, Caryophyllaceae, Malvaceae, Geraniaceae, CEnothe- 

 reae, Rosaceae, and Salicineae. It does not occur at all, or only very ejcQeplionally, 

 in Gramineae, Urticaceae, Labiatae^ Convolvulaceae, Polemoniaceae., Qroi^iitariaceae, 

 Papaveraceae, Cruciferae, Hypericineag, and Papilionaceae. Even genera of the same 

 order or family differ in this respect. Among Caryophyllaceae, the species of 

 Dianthus hybridise easily, those of Silene only with difficulty; among Solanaceae, 

 the species of Nicotiana and Datura have a tendency to produce hybrids, while 

 those of Solatium, Physalis, and Nycandra have not; among Scrophulariaceae, 

 Verbascimi^ and Digitalis, but not Pentstemon, Linaria, or Antirrhirfum ; among 

 Rosaceae, Geum, but not Potentilla. 



' [See also H. Philibert, L'Hybridation dans les Mousses (Grimmia), Ann. des sci. nat. 1873, 

 vol. XVII. p. 225.] 



^ [See also T. Moore on Adiantum farleyense, Journ. Roy. Hort. Soc. new series, I. p. 83 ; 

 Berkeley on Asplenium ebenoides, Scott, ibid. p. 137.] 



=* Nageli, Sitzungsber. der k. bayer. Akad. der Wiss. in Miinchen, Dec. 23, 1865, and Jan. 13, 

 1866, Also Kemer, in Oesterreich. BoV Zeitsch. Wien. XXI. 



* [Siachys ambigua Sm. is considered to be a hybrid betweea S. sylvatica and S. palustris.'] 



' [On hybridity in the genus Verbascum, see Darwin, Journ Linn. Soc. 1868; p. 437'] 



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