INTRODUCTION. 



9 



depressions in these seeds were sometimes mere pitting, 

 as in Victoria Marrow; or they may be so marked that 

 the seed would be described as wrinkled. The latter were 

 especially common in William the First, but microscopic 

 examination showed at once that these seeds are really 

 of the round type. There are, therefore, states Gregory, 

 two entirely different types of wrinkling, and while it is 

 clear that the process by which wrinkling is produced 

 is connected with shrinkage on drying, the regularity of 

 the shrinking of the round type and its irregularity in 

 the two other types can not at present be explained. 

 There occasionally occur among the offspring of hybrids 

 between round and wrinkled types seeds of dubious shape 

 which it is difficult, on superficial examination, to classify 

 as round or wrinkled. The existence of such seeds and 

 types of doubtful shape was taken by Weldon to indicate 

 irregularities of Mendelian segregation and dominance, 

 but Gregory states that no seed has been found which 

 upon histological examination allowed of any doubt as to 

 its true character, and consequently that occasionally 

 pitting and spurious wrinkling must be distinguished 

 from the true wrinkling of the wrinkled types. 



The nature of the starch-grain in the hybrid, and 

 how the characters of the starch-grains segregate, if they 

 do so at all, in subsequent generations, are points which 

 suggested themselves to Darbishire, who states that they 

 are matters on which we are ignorant. He found that 

 the starch-grains of the round pea, such as of the 

 "Eclipse," appear as single potato-shaped grains, with 

 an average length of 0.0322 mm. and an average breadth 

 of 0.0213 mm. The length-breadth-index {i.e., 100 X 

 breadth ~ length) is 66.14. Besides these potato-shaped 

 grains, there are extremely few very much smaller 

 grains which are round. The grains of wrinkled peas 

 like the " British Queen " are compound, each consisting 

 of a number of pieces which vary between 2 and 8. These 

 pieces are held together by a ref rangent yellow substance 

 which does not color blue with iodine, and they are likely 

 to break apart. The commonest types are those with 4, 5, 

 or 6 components; grains with 7 or 8 are rarer; grains 

 with 2 or 3 are intermediate in frequency between those 

 with 4, 5, or 6 on the one hand and 7 or 8 on the other. 

 While the grains with 7 to 8 pieces are not much larger 

 than those with 4, 5, or 6 ; grains with 2 or 3 are always 

 conspicuously smaller than those with 4, 5, or 6. The 

 average length is 0.0269 mm., the average breadth 

 0.0248 mm., and the length-breadth-index is 92.19. In 

 these peas are a number of very small single grains which 

 can be distinguished from the pieces of the compound 

 grains by the fact of their being circular and always 

 smaller than the grains consisting of two pieces. "Very 

 rarely will be found isolated potato-shaped grains. 



The grains of the Pi cotyledons produced by crossing 

 the round with the wrinkled pea are nearly round; the 

 majority of the grains are single and the remainder com- 

 pound; the compoundness exhibited by the compound 

 grains in Fj seeds is intermediate between singleness and 

 the degree of compoundness in the grains of wrinkled 



peas, for while in the latter the number of pieces varies 

 between 2 and 8 and the commonest is 6, in the Fi grain 

 it varies between 2 and 4 and the commonest is 3. The 

 differences in the measurements of the three starches are 

 shown in table 2, by which it will be seen that in shape 

 the Fi grain is intermediate between the potato-shaped 

 grain and the compound grain, but nearer the latter. 



Table 2. 



Darbishire also examined the grains of Fj. These 

 he did not measure, but he states that no differences could 

 be seen between the potato-shaped, compound, and round 

 grains from the three types already described. He notes 

 that the evidence points to the fact that the heterozygote 

 round peas in generations subsequent to Fj are character- 

 ized by the possession of irregular round or round grains, 

 and homozygote round peas by potato-shaped grains. 

 Darbishire records that if the association of round grains 

 with heterozygote round and of potato-shaped grains with 

 homozygote round holds good for the Fj generation, we 

 have a means of distinguishing between DI> round and DR 

 round in Fg, instead of, as at present, having to wait 

 until their progeny are mature in the following year. 

 Another poinit demonstrated by the nature of grains in 

 Fj, and borne out by those of F5, is that the shape of 

 the grain is inherited separately from its composition — 

 if we may use this term to cover the singleness or com- 

 poundness of the grain. In the round pea the grains are 

 single and long; in the wrinkled peas they are- compound 

 and round; in the hybrid they may be either single or 

 compound, but are more round than long. In F5 there 

 are round grains exhibiting much compoundness and 

 others exhibiting little. Possibly there are potato-shaped 

 grains either with no compounds or with few, and inter- 

 mediate grains either with few compounds or with many. 

 The wrinkled peas of this generation contained, as was 

 to be expected, compound grains, but some of them had 

 in addition, very sparingly potato-shaped grains. Dar- 

 bishire also studied the absorptive capacities of the three 

 starches in relation to water. The following facts are 

 summed up from the results of his investigations : 



1. Although roundness is dominant over wrinkled- 

 ness in peas, the round starch-grain of the F, generation 

 is a blend between the type of grain of the round pea 

 (the potato-shaped) and the type of grain of the wrinkled 

 pea (the compound) in respect of three characters: (a) 

 it is intermediate in shape as measured by its length- 

 breadth-index, that of the potato-shaped grain being 

 66.14, that of the compound grains 92.19, and that of the 

 round grain 8.5; (&) it is intermediate in the distribu- 



