VI SEGREGATION IN ASEXUAL REPRODUCTION 173 



factor, we must always reckon with the possibility that mutation, and 

 not segregation, has taken place, as in the case of Mirabilis (p. 182). 

 Suppose a plant with blue (dominant) flowers is crossed with a white 

 variety (recessive) of the same species, the hybrid then contains, according 

 to hypothesis, a pair of homologous chromosomes, one containing the 

 colour factor in its blue condition, which we may call A, the other one 

 the same factor in its white state {a). Now in many instances we know 

 that a was originally derived by mutation from A, and there is no reason 

 to suppose that this may not happen again, leaving the heterozygote 

 with two a chromosomes, i.e. pure white. 



While many cases of supposed vegetative segregation may probably 

 be explained in this way, there can be little doubt that true vegetative 

 segregation does take place as a very rare occurrence. 



A case which can hardly be explained otherwise has been described 

 by Bateson and Pellew (1915). Many varieties of peas (Pisum) produce 

 a small percentage of " rogues," or plants with a somewhat vetch-like 

 habit. The genetic behaviour of heterozygotes between rogues and 

 typical plants is remarkable. As young plants they usually differ very 

 little from the type form, but as they grow older the rogue characters 

 appear in their upper parts, and as adults they are always pure rogues. 

 Moreover, though of heterozygous origin, they produce, when self- 

 fertilized, exclusively rogue offspring. These exhibit the rogue characters 

 even as young plants. 



Thus, as the above-mentioned authors point out, the normal and 

 rogue characteristics of the heterozygote seem to separate during the 

 growth of the plant, the normal characteristics being left behind in 

 the older or lower parts of the plant, leaving purely rogue characteristics 

 in the upper parts, and therefore also in the gametes. 



What the underlying cytological conditions of vegetative segregation 

 may be we do not know, but it may be fairly confidently conjectured 

 that something analogous to the separation of homologous chromosomes 

 in meiosis is concerned in it. 



Summing up, we see that, except as extremely rare exceptions of 

 which nothing is known as to their cytology, segregation in heterozygotes 

 does not take place unless meiosis occurs, and that when meiosis does 

 occur segregation does take place, thus adding direct experimental evidence 

 to the other considerations which lead us to suppose that the separation 

 of homologous chromosomes in meiosis is the cause of segregation. 



It is not to be expected that the distinction between the members of 

 a pair of homologous chromosomes which differ in regard to one or even 

 more of their factors should be visible in all cases under the microscope 

 — though we have seen some examples of visible differences between 



