74 



DIFFERENTIATION AND SPECIFICITY' OF STARCHES. 



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 4, 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 4. 



Darbishire also examined the grains of Fs. These he did not measure, but he states 

 that no differences could be seen between the potato-shaped, compound and round gi-ains 

 from the three types already described. He notes that the evidence points to the fact that 

 the heterozygote round peas in generations subsequent to Fi are characterized by the pos- 

 session 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 F2 generation, 

 we have a means of distinguishing between DD round and DR round in F3, instead of, 

 as at present, having to wait until their progeny are mature in the following year. Another 

 point demonstrated by the nature of grains in Fi, and borne out by those of Fs, is that the 

 shape of the grain is inherited separately from its composition if we may use this term 

 to cover the singleness or compoundness 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 intermediate 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. Darbishire also studied the absorptive capacities of the peas in relation 

 to water. The following facts are summed up from the results of his investigations : 



1. Although roundness is dominant over wrinkledness in peas, the round starch-grain of the Fi 



generation is a blend between the type of grain of the round pea (the potato-shaped) and the 

 type of grain of the wriiikled pea (the compound) in respect of three characters: (a) it is 

 intermediate iu 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 85.5; {b) 

 it is intermediate in the distribution of compoundness, inasmuch as some of the round grains 

 are compound and some single; (c) it is intermediate in the degree of compoundness, inas- 

 much as amongst those round grains which are compound the most common number of 

 constituent pieces is 3, whereas in compound grains it is 6. 



2. In a subsequent generation (Fs) the homozygote round peas contain potato-shaped grains 



and the heterozygote round peas contain round or intermediate gi-ains. But both round and 

 intermediate grains may be associated either with a high or a low degree of compoundness. 



3. Potato-shaped grains occasionally occur in wrinkled peas in F5, and the evidence suggests 



that the existence of these grains in wrinkled peas tends to make them less wrinkled. 



4. A wrinkled pea takes up more water when it germinates than a round one. The hybrid be- 



tween a rounil and a wrinkled pea is intermediate in respect to this character between its 

 two parents. 



5. Rut the intermediateness of the hybrid in absorption capacity is not occasioned by the inter- 



mediateness of the starch-grain of tlie hybrid, because both F2 peas containing round grains 

 and peas containing potato-shapetl grains have the same absorption capacity as the Fi pea. 



