34 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



FlUH'i.ssoij T>A'ri;s()N and (lie iiiilirin^- lalioiirs ol" liis cd-woi luM's ai Oam- 

 brul>^\\ M I'luL'lia 11 analyses of i;ar(lrii planls Iiaw lapidly cxlciidcd oiii' 

 knowlodiit' oi unit -t'aetoi'S. J*j(j[iiipped willi siicli knowledge llu' prac- 

 tical breeder wdl be able io make new eombiualiioiis with a. cerlaiiily 

 undreamed of ten years ai^o, and more important siill, the brccdcM-, 

 by the use of Mendel's law, wdl be able to produce his novelties ready 

 " fixed " without loss of time, and by strict isolation will be able Lo 

 maintain a true stock without the ti'ouble of " rogueing. " It is im- 

 possible to refer in detail to the considerable work that has already 

 been done in discovering unit-factors in garden plants by many experi- 

 menters. The mnnerous experiments with peas, beans, sweet peas, 

 ten-week stocks, snapdragons, prinu'oses, and other plants, by M I'IN.dhjl, 

 Bateson, Punnett, Saunders, Correns, Tschekmak, J on an n sen, 

 Lock, Biffen, Wheldale, Gregory, Siiull, Emerson, Baur, and 

 others are now well known.* These valuable pioneer contributions are 

 however but a beginning, and when we survey the vast extent of the field 

 still unexplored, we realize that a huge harvest of results remains to be 

 reaped by future experimenters. The unit-factors of such valuable 

 garden plants, for instance, as orchids, roses, rhododendrons, chrysan- 

 themums, dahlias, begonias, cinerarias, carnations, pansies, petunias, 

 poppies, clematis, iris, gladiolus, lilies, geraniums, fuchsias, gloxinias, 

 etc., are as yet practically unknown, to say nothing of the more diffi- 

 cult but equally important garden fruits, such as apples, pears, plums, 

 cherries, gooseberries, currants, and strawberries. So far no garden 

 plant can be said to have been thoroughly worked out by Mendelian 

 analysis, but in a few cases great progress has been made, and for 

 the practical guidance of breeders we may refer in some detail to two 

 cases, the snapdragon and the sweet pea, which will serve as an illus- 

 tration of the practical value of unit-factors. We will take the case 

 of the snapdragon first, as the factors so far only refer to the flower 

 colours. 



Unit-Factors in the Snapdragon (Antirrhinum majus). 



For the elucidation of the complicated colours of the modern snap- 

 dragon we are indebted to the exhaustive experiments of Miss Whel- 

 DALE at Cambridge. No less than sixteen thousand plants have been 

 raised and flowered in the course of these experiments. So far. Miss 

 Wheldale has found at least seven unit-factors which go to make up 

 the flower-colours of the snapdragon. All these factors are evidently 

 present in the original wild form, which has self-coloured flowers of 

 a " magenta " hue. Five of these factors may be represented as 

 follow si : — 



Y representing yellow colour in the lips. 



I representing ivory colour in the lips. 



* For references and details see Professor Bateson's Mendel's Princi'ples of 

 Heredity. Camb. Univ. Press, 1909. 



t The remaining two factors are S representing a striped or streaked condition 

 of the intensified magenta, and another factor representing a carmine pink 

 pigment, which is present in the newer varieties of snapdragon known in gardens 

 as 'Rose Dore.' The precise properties of these two factors are not yet fully 

 known, so that for the present they may be omitted. Miss Wheldale states 

 that the S factor behaves as a recessive to the D factor. It would appear there- 



