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.Ä^zt. 1885) obtained hybrid embryos by mixing the oospheres oi Fiicus vesiadostis 

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

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

 A. Braun ( Verjüngu7tg,^.'^2g) mentions hybrids of the Mosses Physcomitriiim pyriforme 

 with Funana hygromeirica, and Physconiitriuni fasciculare with Funaria hygromeln'ca, 

 also Fern hybrids of Gym7wgramme chrysophylla and Gynuiogramme cahmelcEna, 

 Gynuiogramine chrysophylla with G. dislans, and of Aspidiiiin filix-mas with A. 

 spiniilosum. 



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 Kölreuter in the 

 last century, and later by Knight, Gärtner, 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 Liliacece 

 Irideoe, Nyctaginese, Lobeliacese, Solanacese, Scrophularinese, Gesneracese, Primulacese, 

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

 niaceae, (Enotherese, Rosacese, and Salices. Hybridisation of species succeeds not 

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

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

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

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

 oi Silene\M\\}a difficulty; among the Solanaceae the species oi Nicotiana andof Ä7/«ra 

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

 the Scrophularineae the species of Verbasciim and Digitalis, but not those of Pcntas- 

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

 not of Potentilla. 



Hybridisation between different genera has been observed between Lychnis and 

 Silcne, Rhododefidron and Azalea, Rhododendron and Rhodora, Azalea and Rhodora, 

 Rhododendron and Kalmia, Rhododendron and Menziesia, Aegilops and Tritician, 

 Echinocactiis, 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 



