172 CYTOLOGY 



CHAP. 



acteristics do not segregate among their offspring, these all resembling 

 their hybrid parent (Ostenfeld, 1904, and Rosenberg, 1907). Indirect 

 evidence of the absence of segregation in parthenogenesis has also been 

 obtained in the case of the Cladoceran Simocephalus (Agar, 1914). A 

 " population " of females hatched from fertilized eggs was allowed to 

 reproduce itself parthenogenetically. The females included representa- 

 tives of numerous size-types, and having been collected from the same 

 locaHty must have included many heterozygotes between the various 

 types. Now, if segregation were taking place, the resemblance between 

 parent and offspring, as measured by the correlation co-efficient, must 

 get more and more perfect generation after generation in parthenogenesis, 

 since heterozygotes could split into homozygotes, but, in the absence 

 of syngamy, these could not recombine into heterozygotes. Thus the 

 original mixed population must get more and more nearly homozygous, 

 and the correlation between parent and offspring must consequently 

 rise from generation to generation. This correlation co-efficient was 

 measured for the first five parthenogenetic generations from the fertilized 

 eggs and was found to remain practically constant — at any rate it showed 

 no sign of increasing. Hence we conclude that no segregation was taking 

 place. 



(3) Segregation and Bud-Variation 



A few cases have been described where segregation appears to take 

 place in vegetative (somatic) growth in plants. The most famous case 

 is the laburnum Cytisus adami, which is a hybrid between the purple 

 C. purpureas and the yellow C. laburnum. C. adami has dingy red flowers 

 which are sterile. It occasionally, however, gives rise to pure, or almost 

 pure, branches or single flowers of C. laburnum and C. purpureus. The 

 flowers on these branches are fertile, and give rise to C. laburnum and 

 C. purpureus plants respectively (accounts differ as^ to whether these 

 plants are quite pure or show traces of the other species). 



Many cases have been described where a dominant plant hybrid 

 has produced, by bud-variation, branches or flowers with the character- 

 istics of the recessive parent. Before these can be put down as examples 

 of vegetative segregation, however, two possibiUties have to be taken 

 into account. Firstly, do the recessive flowers breed true to the recessive 

 character ? Otherwise the appearance of the recessive character in one 

 part of the plant may be due to some somatic condition preventing the 

 dominance of the normally dominant characteristic — and many cases 

 are known where dominance of one or other characteristic in a hybrid 

 is affected by somatic conditions (for examples, see Cramer, 1907). 

 Secondly, even if the recessive bud-variations do breed true to the 

 recessive character, showing that they no longer contain the dominant 



