PRACTICAL ASPECTS IN HEREDITY. O 



For example, in a case that I see a great deal of, in the sweet pea, you may 

 by crossing two sweet peas produce the old purple sweet pea with chocolate 

 colored standards and purple wings. That purple sweet pea so produced will 

 not breed true. The old purple pea of the gardeners was a pure pea and 

 would perpetuate itself truly from seed; but the purple pea produced as a 

 heterozygote form will not breed pure, but will split up into the components 

 which produced it. So that we recognize that there is a new form, a hetero- 

 zygote form, which, though it may resemble some pure form, will not breed 

 true. This is a case that may not interest the seedsman, because he does not 

 \vant the old sweet pea. Nevertheless, in his fertilizations he may produce 

 another new form which he does want, and after all his laborious selection 

 he may find it is only a heterozygote which will never breed true. 



It is a curious and unexplained fact constituting one of the most fertile 

 fields of inquiry that when dissimilar gametes meet they should so often 

 produce an ancient form. That is what we now recognize as the rationale of 

 Darwin's "reversions on crossing." When Darwin crossed his pigeons he 

 brought back an old form ; and so in crossing many plants you can get back a 

 reversionary form by uniting two dissimilar gametes. 



In my own experience a most extraordinary case of this nature has 

 occurred. When the Mendelian discoveries were first announced it was ob- 

 viously desirable to cross two varieties differing in some visible character of 

 the gametes (whether alike in other respects or not). By such means we might 

 hope to make visible that mixture of dissimilar gametes which must certainly 

 occur in Mendelian hybrids. Unfortunately the actual gametes of flowering 

 plants are not adapted to this experiment, but the nearest things to them are 

 the pollen grains. So I cast about for a case of visible variation in pollen. 

 By good fortune I found them at once in a certain sweet pea. 



Ordinary sweet peas have their pollen grains elongated, with three pores. 

 The white variety known as Emily Henderson, an American sort about eight 

 to ten years old, usually has pollen grains which, when treated with acids, etc., 

 are seen to be roughly spherical, with only two pores. Various grains of inter- 

 mediate types are found from time to time in the pollen of E. Henderson, 

 and the round grains not very rarely have three pores. But the pollen of a 

 round-pollened plant can generally be distinguished immediately from that of a 

 long-pollened plant. 



Proposing, then, to cross E. Henderson with sweet peas having typical 

 pollen, I sowed a quantity of that variety. But when the plants flowered I 

 discovered that, though a majority of the Hendersons had round pollen, a 

 few, though otherwise indistinguishable from the others, had nevertheless 

 long pollen, exactly like a common sweet pea. I then crossed the round 

 pollened Henderson with the long, and vice versa. The same experiment was 

 also made independently by Miss E. R. Saunders. Every seed then produced 

 (from four capsules') has given a plant with chocolate-purple standards and 

 blue-purple wings! There are many details respecting this remarkable case 

 which I hope ere long to publish, but I mention it now as illustrating in a 

 striking way how paradoxical are the phenomena empirically produced by the 

 experimental breeder, and how puzzling are these heterozygous forms. 



I may say that my experiment entirely failed to fulfil its original purpose. 



