HYBRIDITY 373 



other plant. It is possible that by transferring a drop of the substance secreted 

 by the stigma of the pollen-bearing parent to the stigma of the other plant 

 germination of the pollen-grains may be induced. 



When the hybridization is successful seeds are formed which resemble in 

 form, colour, and size the normal seeds of the mother-plant ; the fruits also are 

 uninfluenced by the male parent. A variation can ensue only where the contents 

 of the pollen-grain come immediately into action ; that is to say, where the two 

 generative nuclei have fused with the essential nuclei of the embryo-sac, i. e., the 

 endosperm on the one hand, and the embryo and the plant arising from it on 

 the other. At present we must omit any consideration of the endosperm of the 

 hybrid, and deal only with the hybrid proper. No general rule can be laid down 

 as to the appearance of the hybrid. Many interspecific hybrids show a struc- 

 ture exactly intermediate between that of the two parents, as Kolreuter 

 described it in the case of the first recorded ' botanical cross ' — the hybrid 

 Nicotiana rustica^ x N. paniculata o^: — ' I was gratified to find that the hybrid 

 took a median place between the two parents not only in the arrangement of the 

 branches, and in the position and colour of the flowers, but also in all the parts 

 of the flower (the stamens alone excepted), which exhibited an almost geo- 

 metrical mean.' 



Horticulturists, in their researches on hybrids, usually start with the 

 assumption that the hybrids will show characters intermediate between those 

 of the parents, but it is now becoming more and more evident that this is only 

 one of many possibilities. In hybrids between members of closely-related 

 races intermediate characters are often wanting ; for instance, the hybrid 

 obtained by crossing a red with a white-flowered pea is not light red, but red, 

 like one of the parents. Although the white-flowered pea has yellow cotyledons 

 and the red-flowered one has green, the hybrid has yellow cotyledons. From 

 this example it is clear that in the hybrid the one parent does not simply pre- 

 dominate over the other, but that the separate characters of the two parents 

 struggle with each other, one parent being eventually victorious with regard to 

 one character the other in another. In this connexion we may, with Correns, 

 speak of dominant and recessive characters, and we may designate hybrids with 

 such strongly-marked characters as heterodynamic, in contradistinction to homo- 

 dynamic forms, which exhibit characters more or less exactly intermediate. A 

 further complication not infrequently arises, viz. in certain hybrids it cannot 

 be determined once and for all which character is dominant and which reces- 

 sive ; the result may be different in each individual case where the characters 

 are brought into conjunction. Individuals resulting from a single bastard cross- 

 pollination may differ, and in one or in all characters may resemble at one time 

 the mother at other times the father (many species of Hieraciitm, Mendel, 1870), 

 or some of the hybrids may partly resemble the mother only or partly the father 

 only (strawberry, Millardet, 1894). Finally, in other cases still, the decision 

 as to which character shall dominate, or how vigorously it shall show itself is by 

 no means determined by fertilization itself, for individual branches, tissues, 

 cells, or even cell-parts belonging to one and the same hybrid may show varia- 

 tions. Examples of such ' mosaic hybrids ' were found by N.\udin (1862) and 

 in the hybrid Datura laevis $x Z). stramonium, o^, which produced, in addition to 

 fruits with small spines, intermediate, therefore, between the large-spined 

 D. stramonium and the spineless D. laevis, fruits which were smooth on one 

 side and spiny on the other. 



Although the hybrid at first sight often appears as a new creation, more 

 careful study shows that it is only the combination of characters in it that is 

 new ; absolutely new characters do not seem to arise in hybrids, although there 

 are certainly exceptions. The hybrid between the green-stemmed, white-flowered 

 species Datura ferox and D. laevis has brown stems and violet flowers ; it would 

 appear in this case as if a new character, the formation of a new pigment, had 



