Chapter XIX: Appendix

As some readers may possibly care to repeat Mendel's experiments for themselves, a few words on the methods used in crossing may not be superfluous. The flower of the pea with its standard, wings, and median keel is too familiar to need description. Like most flowers it is hermaphrodite. Both male and female organs occur on the same flower, and are covered by the keel. The anthers, ten in number, are arranged in a circle round the pistil. As soon as they are ripe they burst and shed their pollen on the style. The pollen tubes then penetrate the stigma, pass down the style, and eventually reach the ovules in the lower part of the pistil. Fertilisation occurs here. Each ovule, which is reached by a pollen tube, swells up and becomes a seed. At the same time the fused carpels enclosing the ovules enlarge to form the pod. When this, the normal mode of fertilisation, takes place, the flower is said to be SELFED.

In crossing, it is necessary to emasculate a flower on the plant chosen to be the female parent. For this purpose a young flower must be taken in which the anthers have not yet burst. The keel is depressed, and the stamens bearing the anthers are removed at their base by a {188} pair of fine forceps. It will probably be found necessary to tear the keel slightly in order to do this. The pistil is then covered up again with the keel, and the flower is enclosed in a bag of waxed paper until the following day. The stigma is then again exposed and dusted with ripe pollen from a flower of the plant selected as the male parent. This done, the keel is replaced, and the flower again enclosed in its bag to protect it from the possible attentions of insects until it has set seed. The bag may be removed in about a week after fertilisation. It is perhaps hardly necessary to add that strict biological cleanliness must be exercised during the fertilising operations. This is readily attained by sterilising fingers and forceps with a little strong spirit before each operation, thereby ensuring the death of any foreign pollen grains which may be present.

The above method applies also to sweet peas, with these slight modifications. As the anthers ripen relatively sooner in this species, emasculation must be performed at a rather earlier stage. It is generally safe to choose a bud about three parts grown. The interval between emasculation and fertilisation must be rather longer. Two to three days is generally sufficient. Further, the sweet pea is visited by the leaf-cutter bee, _Megachile_, which, unlike the honey bee, is able to depress the keel and gather pollen. If the presence of this insect is suspected, it is desirable to guard against the risk of admixture of {189} foreign pollen by selecting for pollinating purposes a flower which has not quite opened. If the standard is not erected, it is unlikely to have been visited by _Megachile_. Lastly, it not infrequently happens that the little beetle _Meligethes_ is found inside the keel. Such flowers should be rejected for crossing purposes.

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INDEX

_Abraxas grossulariata_, 99
"Acquired" characters, 14
Adaptation, 143
Agouti mice, 50
Albino mice, 50
Albinos, nature of, 53
_Amauris_, 144
Analysis of types, 156
Ancestral Heredity, Law of, 13
Andalusian fowls, 70
Axil colour in sweet peas, 93

Bateson, W., 14, 29, 55, 116, 132, 141
Biffen, R. H., 157
Blue Andalusian fowls, 71
Brachydactyly, 171
Bryony, 120
Bush sweet peas, 63

Castle, 132
Cattle, horns in, 86, 166
Colour, nature of, in flowers, 48
Colour-blindness, 117
Combs of fowls, 33, 43
Correns, C., 29, 120
Coupling of characters in gametes, 93
Cuénot, 50, 119
"Cupid" sweet peas, 62
Currant moth, 99

Darwin, C., 10, 65, 147, 163
De Vries, H., 15, 29, 141
Discontinuity in variation, 14
Dominant characters, 18
Doncaster, L., 99
Drinkwater, H., 172
Dutch rabbits, 60

Eggs, 2
Environment, influence of, 137
_Euralia_, 144
Evolution, 10, 85, 139
Eye, in primulas, 55
Eye-colour, in man, 176

Factor, definition of, 31
Factors, interaction of, 42
Fertilisation, 3
Fertilisation, self- and cross-, 163
Fixation of varieties, 153
Fluctuations, 138
Fowls, coloured from whites, 49, 73

Galton, 13, 179
Gametes, nature of, 6
Gregory, R. P., 55, 93

Hæmophilia, 176
Hardy, G. H., 147
Heterozygote, definition of, 28
Heterozygote, of intermediate form, 68
_Hieracium_, 27, 132
Himalayan rabbits, 60
Homostyle primulas, 56
Homozygote, definition of, 28
Hooded sweet peas, 89
Horses, bay and chestnut in, 167
Hurst, C. C., 62, 176, 180

Immunity in wheat, 158
Individuality, 135
Inhibition, factors for, 74, 108
Intermediates, 125
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Johannsen, W., 160

Lop-eared rabbits, 132

Mendel, 8, 17, 26, 132
Mental characters, 180
Mice, inheritance of coat colour in, 50
Mimicry, 143
_Mirabilis_, 151
Morgan, T. H., 116
Mulattos, 129
Mutation, 83, 138

Nägeli, C., 26
Natural selection, 11, 140, 142, 149
Nettleship, E., 175
Night-blindness, 175

_Pararge egeria_, 132
Parkinson, J., 122
Pea comb, 33
Peas, coloured flowers in, 24
Peas, tall and dwarf, 18
Pigeons, 86
Pin-eye in primulas, 55
_Pisum_, 17
Primulas, 31, 55, 68, 93
Pollen, 3
Pollen of sweet peas, 92
Pomace fly, 115
Population, inheritance of characters in a, 147
Presence and Absence theory, 35
Pure lines, 162
Purity of gametes, 24
Purity of type, 155

Rabbits, 53, 60
Ratios, Mendelian--
3 : 1, 20
9 : 3 : 3 : 1, 25, 34
9 : 3 : 4, 51
9 : 7, 49
Ray, John, 143
Recessive characters, 19
Repulsion between factors, 90
Reversion, 59, 165
in rabbits, 59
in sweet peas, 62
in fowls, 65
in pigeons, 65
Rose comb, 33

Saunders, E. R., 54, 122
Seeds, nature of, 4
Segregation, 22
Selection, 162
Sheep, horns in, 76
Silky fowls, 30, 105
Single comb, 32
Species, nature of, 150
Species, origin of, 11
Speckled wood butterfly, 132
Spermatozoa, 3
Sports, 147
Staples-Browne, R., 66
Sterility, 151
Sterility in sweet peas, 93
Stocks, double, 122
Stocks, hoariness in, 54
Sweet pea, colour in, 44, 79
history of, 82
inheritance of hood in, 89
inheritance of size in, 62

Telegony, 167
Thrum-eye in primulas, 55
Toe, extra toe in poultry, 76
Tschermak, E., 29

Unit-character, definition of, 31

Variation, 14, 137, 139

Walnut comb, 33
Weismann, A., 13
Wheat, beard in, 74
experiments with, 157
White, dominant in poultry, 72
Wilson, J., 168

Yellow mice, 119

Zygotes, nature of, 5

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Notes

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[1] Cf. note on p. 171.

[2] It has been found convenient to denote the various generations resulting from a cross by the signs F_1, F_2, F_3, etc. F_1 on this system denotes the first filial generation, F_2 the second filial generation produced by two parents belonging to the F_1 generation, and so on.

[3] Hurst's original cross was between a Belgian hare and an albina Angora, which turned out to be a masked Dutch.

[4] The Spot is an almost white bird, the colour being confined to the tail and the characteristic spot on the head.

[5] The reader who searches florists' catalogues for these varieties will probably experience disappointment. The sweet pea has been much "improved" in the past few years, and it is unlikely that the modern seedsman would list such unfashionable forms.

[6] It is to be understood that wherever a given factor is present the plant may be homozygous or heterozygous for it without alteration in its colour.

[7] It should be mentioned that as the shape of the pollen coat, like that of the seed coat, is a maternal character, all the grains of any given plant are either long or else round. The two kinds do not occur together on the same plant.

[8] For the most recent discussion of this peculiar case the reader is referred to Professor Castle's paper in _Science_, December 16, 1910.

[9] _Paradisus Terrestris_, London, 1629, p. 261.

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Transcriber's note:

Corrections made to printed original.

Page 36, "two sorts, RP and Rp": 'PR and Rp' in original.

Page 51, "9 contain both C and G": 'c and G' in original.

Page 184, "in the simplest cases": 'simples' in original.

Footnote 3, "turned out to be a masked Dutch": 'turned to
out be' in original.