gi6 
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 Kalmia, Rhododendron and Menziesia^, 
jEgilops and Triii'cum, and between Echmocac/us, Cereus, and Phjllocactus, 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^, AnagaUis arvenis and ccBrulea, Primula officinalis and elatior, 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 jEgilops ovala with Trilicuni vulgare, Lychnis diurna with 
Z. Flos-cuctdi, Cereus speciosissimus with Phyllocactus Phyllanlhus, 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. anguslifolia, var. latifolia with var. liiioralis, &c. 
3. 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 Fucus vesiculosus 
produces hybrids with the antherozoids of F. serrahis, 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. Kölreuter 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 grt)w vigorously, but are them- 
selves fertile. The lowest degree of the action of pollen of a different kind consists 
in 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 of Bryanthus {Menziesia) empetriformis, supposed to be fertilised by 
the pollen of Rhodothamnus (Rhododendron) Chamcecistus. It is figured tinder the name of Bryanthus. 
erectns 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.] 
