8i8 PHENOMENA OF SEXUAL REPRODUCTION. 



Verbascum ^ and Digitalis, but not Pentstemon, Linaria, or Antirrhinum ; among 

 Rosaceae, Geum, but not Potentilla. 



Hybridisation between species belonging to different genera has been observed 

 between Lychnis and Silene, Rhododendron and Azalea, Rhododendron and 

 Rhodora, Azalea and Rhodora, Rhododendron and Kalmia, Rhododendron and 

 Menziesia^ ^Egilops and Triticum, and between Echinocactus, Cereus, and Phyl- 

 locactus, to which must be added a few wild forms which appear to be genus- 

 hybrids. 



Besides the near genetic relationship, the possibility of the production of 

 hybrids depends also on a certain relationship between the parent-plants, which is 

 manifested only in the result of hybridisation, and which Nageli calls ' Sexual Affinity.' 

 This kind of affinity is not always concurrent with the external resemblance of the 

 plants. Thus, for example, hybrids have never been obtained between the apple 

 and pear^, Anagallis arvensis and ccerulea, Primula officinalis and elalior, or Nigella 

 damascena and saliva, nor between many other pairs of species belonging to the same 

 genus which are very nearly allied to one another ; while in other cases- very dis- 

 similar forms unite, as Jigilops ovala with Trilicum vulgare, Lychnis diurna with 

 L. Flos-ciictili, Cereus speciosissimus with Phyllocaclus PJiyllanlhus, the peach with the 

 almond. A still more striking proof of the difference between sexual and genetic 

 affinity is afforded by the fact that varieties of the same species will sometimes be 

 partially or altogether infertile with one another, as e.g. Silem inflala var. alpina with 

 var. angusli/olia, var. latifolia with var. lilloralis, «fec. 



When a sexual union is possible between two species A and B, A can usually 

 produce hybrids when fertilised by the pollen of B, and B when fertilised by the 

 pollen of A (reciprocal hybridisation). But there are cases in which A can only be 

 the male and B only the female parent plant, the pollination of A by B yielding no 

 result. Thus Thuret found, as has already been mentioned, that Fuctis vesiculosus 

 produces hybrids with the spermatozoids of F. serralus, while the oospheres of the 

 latter species could not be fertilised by the spermatozoids of the former. Gartner 

 states that Nicotiana paniculata produces seeds when acted on by the pollen of 

 N. Langsdorfii, while the latter does not under the influence of the pollen of the 

 former. Kolreuter easily obtained seeds of Mirahilis Jalappa with the pollen of 

 M. longiflora, while more than two hundred experiments on pollinating the latter by 

 the former species extending over eight years produced no result. 



Sexual Affinily presents a great variety of gradations. At one extreme we have 

 complete infertility under the influence of the pollen of another variety or species, the 

 pollen-tubes not even entering the stigma, and the pollinated flower behaving pre- 

 cisely as if no pollen had reached it ; the other extreme is shown in the production 



^ [On hybridity in the genus Verbascum, see Darwin, Journ. Linn. Soc. 1868, p. 437. — Ed.] 

 "^ [The history of the plant which is here intended is given in the Botanical Gazette, vol. Ill, 

 p. 82. It was raised from seed of Bryanthus {Menziesia) empetriformis, supposed to be fertilised by 

 the pollen of Rhodothanuins (Rhododendron) ChamcEcistus. It is figured under the name of Bryanthiis 

 erechis in Paxton's Flower Garden, vol. I, 1. 19 ; but it agrees well with specimens of its female parent 

 from the Rocky Mountains, and is probably therefore not a hybrid at all. — Ed.] 



^ [An instance to the contrary is recorded in the Proc. Acad, Philadelphia, 1S71, vol, I, p. 10. 

 —Ed.] 



