yi6 PHENOMENA OF SEXUAL REPRODUCTION. 



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 Kalmm, Rhododendron and Menziesia^, 

 jEgtIops and Triti'cum, and between Echinocaclus, Cercus, and Phyllocactus, to which 

 must be added a few wild forms which appear to be genus-hybrids. 



2. 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 arvcnis and ccBrulea, 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 ^gilops ovata with Triticum vulgare, Lychnis diurna with 

 Z. Flos-cuculi, Cereus speciosissimus with Phyllocactus Phyllanihus, 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. Silene inflata var. alpina with 

 var. angustifolia, var. latifolia with var. liiloralis, &c. 



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

 produce hybrids when fertihsed 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 Fucus vesiculosus 

 produces hybrids with the antherozoids of F. serratus, while the oospheres of the 

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

 states that Nicotiana paniculata produces hybrid 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 Jalapa 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. 



4. Sexual affinity 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 precisely as if no pollen had reached it ; the other extreme is shown in 

 the production of numerous hybrids, which not only grow vigorously, but are them- 

 selves fertile. The lowest degree of the action of pollen of a different kind consists 

 ih various changes taking place in the parts of the flower of the mother-plant, the 

 ovary or even the ovules also growing, without any embryo being produced. 

 A higher degree is manifested in the production of ripe normal fruits and seeds 



' [The history of the plant which is here intended is given in the Botanical Gazette, vol. III. 

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

 the pollen of Rhodothammis (Rhododendron) Chamcecisttis. It is figured under the name of Brynnthus 

 erechis in Paxton's Flower Garden, vol. I. t. 19; but it agrees well with specimens of its female 

 parent from the Rocky Moimtains, and is probably therefore not a hybrid at all.] 



^ [An instance to the contrary is recorded in the Proc. Acad. Philadelphia, 1871, vol. I. p. 10.] 



