;o6 SCIENCE PROGRESS 



There are then two pairs of differentiating characters to be 

 considered— the presence and absence of fertile lateral florets and 

 the black or white colour. 



The cross was made in both directions, H. Steiidelii being 

 used as both male and female parent. The resulting hybrids 

 resembled one another and were very similar to H. Steudelii 

 inasmuch as they were black in colour and the lateral florets 

 were grainless. A closer examination showed, however, that 

 these florets were slightly larger than in that parent, and on dis- 

 section they were found to contain stamens. In this respect, 

 then, the hybrid differs from either parent. On sowing the 

 grains produced as the result of self-fertilisation they produced 

 plants in which the lateral florets were either hermaphrodite, 

 staminate, or completely sexless. Both black and white forms 

 of each of these types occurred. For the present the colour 

 characters can be neglected and attention confined solely to the 

 development of the lateral florets. The statistics showed that 

 the three classes were present in the ratio of i : 2 : 1. Numbers 

 of plants of each type were harvested separately and their grain 

 again sown, with the result that all those with hermaphrodite 

 and all those with sexless lateral florets were found to breed 

 true to these particular characters. Those with staminate lateral 

 florets, on the contrary, in all cases gave a mixed offspring 

 in which the three groups with hermaphrodite, staminate, and 

 sexless lateral florets again appeared in the proportions of 1 : 2 : 1. 



This pair of characters provides a simple example of Men- 

 delian segregation, and they may be taken as a starting-point for 

 explaining the details of Mendel's discoveries from. Calling the 

 hermaphrodite character A and the sexless lateral a, then the 

 hybrid with staminate laterals produced when A and a meet will 

 be Aa. The heterozygote Aa produces gametes which are either 

 A's or a's, not a blend of the two. Where gametes carrying 

 the A character meet, the embryo which is produced can give 

 rise solely to plants of this character — that is, with hermaphro- 

 dite lateral florets : similarly, where a and a meet, the plant 

 which results from this combination can only bear sexless 

 lateral florets. Where A and a meet, as in the original crossing, 

 the form with staminate lateral florets is again the result. No 

 other combinations are possible besides AA, Aa, aA, and aa, 

 and as Aa and aA are known, from the reciprocal crosses, to 

 be identical, we have a complete explanation of the fact that the 



