780 LECTURE XLIV. 



different species of a genus, or two species from different genera have united 

 sexually, the resultant hybrid product may be designated as a variety-hybrid, a 

 species-hybrid, or a genus-hybrid. 



Of Cryptograms only a few hybrids are known with certainty. Thuret (Ann, des 

 Sc. Nat. 1885) obtained hybrid embryos by mixing the oospheres oi Fucus vesiculosa 

 with the zoosperms of Fucus serralus. In a few other groups of Cryptogams forms 

 have been found which from their properties are referred to a hybrid origin. Thus 

 A. Braun (F«?7«»^««^,p.329) mentions hybrids of the Mosses Physcomitrium pyriforme 

 with Funaria hygrometrica, and Physcomitrium fascicular e with Funaria hygrometrica, 

 also Fern hybrids of Gymnogramme chrysophylla and Gymnogramme calomelcena, 

 Gymnogramme chrysophylla with G. distans, and of Aspidium filix-mas with A. 

 spinulosum. 



The hybrids of Phanerogams obtained by artificial transference of pollen are 

 however preferably valuable for scientific considerations regarding hybridisation, which 

 at the same time render clearer the meaning of sexuality generally. Naegeli (i) has 

 collected the results of many thousands of hybridisations, made by Kolreuter in the 

 last century, and later by Knight, Gartner, Herbert, Wichura and other observers. 

 From this critical survey of Naegeli's I take the following statements as examples. 



(i) Only such plants as are systematically nearly allied can form hybrids with 

 one another. Hybridisation is effected most easily and completely as a rule between 

 different varieties of the same species ; the production of hybrids is more difiicult, 

 though in many cases possible, between two different species of the same genus. 

 Only a few cases are known of hybrids between species which are placed in different 

 genera, and it is probable that such species, one of which successfully fertilises the 

 other, should be placed together in the same genus. The ability of species to form 

 hybrids exists moreover to very different degrees in different orders, families, and genera 

 of Angiosperms. The following are as a rule favorable to hybridisation : the Liliaceae 

 Irideoe, Nyctagineae, Lobeliacese, Solanacese, Scrophularinese, Gesneracese, Primulacese, 

 Ericaceae, Ranunculacese, Passiflorese, Cactacese, Caryophyllacese, Malvaceae, Gera- 

 niaceae, (Enothereae, Rosaceae, and Salices. Hybridisation of species succeeds not 

 at all, or only exceptionally, among the Gramineae, Urticaceae, Labiatae, Convolvu- 

 laceae, Polemoniaceae, Ribesiaceae, Papaveraceae, Cruciferae, HyperJcineae, Papilio- 

 naceas. Moreover the genera of the same order or family behave differently. 

 Among the Caryophyllaceae the species of Dianthus may be easily hybridised, those 

 of &/e«^with difficulty; among the Solanacese the species oi Nicotiana a.nd of Datura 

 are prone to hybridise, but not those of Solanum, Physalis and Nycandra ; and among 

 the Scrophularinese the species of Verbascum and Digitalis, but not those of PentaS' 

 temon, Linaria, and Antirrhinum ; and among Rosaceae the species of Geum, but 

 hot of Potentilla. 



Hybridisation between different genera has been observed between Lychnis and 

 Silene, Rhododendron and Azalea, Rhododendron and Rhodora, Azalea and Rhodora, 

 Rhododendron and Kalmia, Rhododendron and Menziesia, Aegilops and Triticum, 

 Echinocactus, Cereus, and Phyllocactus, besides a few wild forms which are apparently 

 to be explained as genus-hybrids. 



(2) Besides the close systematic relationship, a certain relation of the plants con- 

 cerned to one another in addition decides as to the possibility of the formation of 



