HYBRIDISA TION. 
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 sei. nat., 1855) obtained hybrid plants by bringing 
antherozoids of Fucus serratus into contact with oospheres of F. veskulosus. 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 (Verjüngung, 
p. 329) adduces instances of hybrids between Mosses ^ PhyscGmitrium pyriforme and 
Funaria hygrofjietrica, and between Physcomitrium fasciculare and Funaria hygro- 
metrica, and between the following species of Ferns — Gymnogramme chrysophylla 
and G. calomelana, G. chrysophylla and G. distans, and Aspidhim Filix-mas and 
A . spmulosum ^. 
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. Nägeli has collected the results of 
many thousand experiments on hybridisation made by Kölreuter 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 fonns 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 
Angiosperms. The phenomenon is frequent among Liliaceae, Iridese, Nyctaginese, 
Lobeliacese, Solanaceae, Scrophulariacese, Gesneraceae, Primulaceae, Ericaceae, Ranun- 
culaceae, Passifloraceae, Cactaceae, Caryophyllaceae, Malvaceae, Geraniaceae, (Enothe- 
reae, Rosaceae, and Salicineae. It does not occur at all, or only very exceptionally, 
in Gramineae, Urticaceae, Labiatae", Convolvulaceae, Polemoniaceae, Grossulariaceae, 
Papaveraceae, Cruciferae, Hypericineae, 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 Nicoiiana and Datura have a tendency to produce hybrids, while 
those of Solanum^ Physalis, and Nycandra have not ; among Scrophulariaceae, 
Verbascum^ and Digitalis, but not Pentstemon, Linaria ^ or Antirrhitium ; among 
Rosaceae, Geum, but not Potentilla. 
' [See also H. Philibert, L'Hybiidation dans les Mousses {Grimmia), Ann. des sei. 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 ehenoides, Scott, ibid. p. 137.] 
^ Nägeli, Sitzungsber. der k. bayer. Akad. der Wiss. in München, Dec. 23, 1865, and Jan. 13, 
1866. Also Kemer, in Oesterreich. Bot. Zeitsch. Wien. XXI, 
* [Slachys ambigtm Sm. is considered to be a hybrid between S. sylvatica and S. palustris.'] 
^ [On hybridity in the genus Verhasctim, see Darwin, Journ. Linn. Soc. 1868, p. 437.] 
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