EXPERIMENTS IN PLANT HYBEIDISATION. 



21 



other with an egg cell a, and so likewise one pollen cell a, will unite with 

 an egg cell A, and the other with egg cell a. 



Pollen cells A 

 1 



Egg cells A 



The result of the fertilisation may be made clear by putting the signs 

 for the conjoined egg and pollen cells in the form of fractions, those 

 for the pollen cells above and those for the egg cells below the line. 

 "We then have 



A A a , a 

 A"^a"^A"^a' 



In the first and fourth factor the egg and pollen cells are of like 

 kind, consequently the product of their union must be constant, ^ iz. A 

 and a ; in the second and third, on the other hand, there again results a 

 union of the two differentiating characters of the stocks, consequently the 

 forms resulting from these fertilisations are identical with those of the 

 hybrid from which they sprang. There occurs accordingly a repeated 

 hybridisation. This explains the striking fact that the hybrids are able 

 to i)roduce, besides the two parental forms, offspring which are like them- 

 A a 



selves ; - and both give the same union Aa, since, as. already remarked 

 a A. 



above, it makes no difference in the result of fertilisation to which of the 

 two characters the pollen or egg cells belong. We may write then — 



^_+^4-^-4.a=A + 2 Aa + a. 

 A a A a 



This represents the average result of the self-fertilisation of the hybrids 

 when two differentiating characters are united in them. In solitary flowers 

 and in solitary plants, however, the ratios in which the forms of the series 

 are produced may suffer not inconsiderable fluctuations. Apart from 

 the fact that the numbers in w^hich both sorts of egg cells occur in the 

 seed vessels can only be regarded as equal on the average, it remains purely 

 .a matter of chance which of the two sorts of pollen may fertilise each 

 separate egg cell. For this reason the separate values must necessarily be 

 subject to fluctuations, and there are even extreme cases possible, as were 

 described earlier in connection with the experiments with the form of 

 the seed and the colour of the albumen. The true ratios of the numbers 

 J3an only be ascertained by an average deduced from the sum of as many 

 single values as possible ; the greater the number the more are merely 

 jchance elements eliminated. 



The developmental series for hybrids in which two kinds of difl'erentiat- 

 jng characters are united contains among sixteen individuals ninedifterent 

 forms, viz., AB + Ab + aB + ab + 2 ABb + 2 aBb + 2 AaB + 2 Aab + 4 AaBb. 

 Between the difterentiating characters of the original stocks Aa and Bb four 

 constant combinations are possible, and consequently the hybrids produce 

 the corresponding four forms of egg and pollen cells AB, Ab, aB, ab, and 

 «each of these will on the averaire Assure four times in the fertilisation, 



