202 Double Flowers and Sex-Linkage in Begonia 



and its descendants, the pollen bears white flower colour and femaleness, 

 the factors for blue and for the hermaphrodite condition being carried 

 by the ovules (Pellew). In this case as in Begonia the sex-linkage was 

 not general but special to a particular plant and its descendants. Of 

 these examples the plastid-colour is the only one in which the converse 

 combination has yet been built up, though perhaps the others may 

 hereafter be obtained ^ 



The condition in Oenothera " velutina " described by de Vries must 

 be very similar, the recessive dwarf character being carried by the 

 pollen. In the corresponding case of Oenothera " laeta" the evidence 

 also points to the pollen being all dwarf, and to the existence of a mix- 

 ture of tails and dwarfs among the ovules, in spite of which the plants 

 do not throw dwarfs on self- fertilisation. This absence of dwarfs on 

 selfing constitutes a puzzle exactly like that of the presence of slightly 

 petalodics in Davisii x double </ ^ 



When in hermaphrodite flowers the male and female sides are 

 genetically distinct we feel fairly sure that the segregation of these 

 allelomorphs occurs not later than the formation of the anther-rudiments, 

 but in B. Davisii it presumably happens even earlier and not later than 

 the formation of the male flowers. Those who incline to regard the 

 reduction division as the stage at which alone segregation can be effected 

 may no doubt be tempted to suggest that in B. Davisii, for instance, 

 pollen grains bearing the dominant factor are in reality formed but in 

 some unexplained way fail to take part in fertilisation. As a mere 

 suggestion of a possibility that theory cannot as yet be absolutely 

 excluded, but in this special example it is more than usually difficult to 

 accept, since the pollen of B. Davisii is to the eye exceedingly uniform 

 and regular. There are none of the shrivelled grains which are gene- 

 rally looked upon as the bearers of missing elements. Though less 

 significant, the absence of seeds partially defective is also noticeable. 



In applying the term sex-linkage to such cases as this I am following 



1 Since in the original form the ovules were mixed and the pollen was all recessive, 

 the " converse " might appear in one of two forms. Either (1) the ovules might be all 

 dominant and the pollen mixed ; or (2) the ovules might be mixed and the pollen all 

 dominant. As Miss Saunders's plants were tested by self-fertilisation and not by crosses 

 with recessives it cannot yet be declared which of the above possible constitutions they 

 possessed, but she considers there is a presumption that they were really arranged on the 

 second of the two plans. (See Saunders, Jour. Gen. iv. pp. 332 and 359 and compare 

 Pellew, Jour. Gen. vi. p. 320, &c.) 



2 For a discussion of these Oenothera cases see W. Bateson, Problems of Genetics, 

 1913, p. 113. 



