NECESSITY FOR INBREEDING. 25 



• 



yield of seed the inbred seed equaled or exceeded in quantity the 

 cross-fertilized seed in the \ ariety. In the case of hybrid^^ or in 

 the crossing of two distinct strains or varieties the yield of seed, 

 as well as rate of growth of the hybrid plants, was greater than that 

 of the inbred seed and plants. It is true that some of the improved 

 inbred strains produce but little seed compared with unimproxed 

 types of the ^;ame variety. Inbred strains have been selected tor 

 increased yield and number of leaves, which seem to be correlated 

 with lessened seed production. The same correlation holds true 

 where open-fertilized strains have been selected for increased num- 

 ber and total yield of leaves. 



The rate of germination of the inbred in comparison with the 

 cross-fertilized seed was thought to be slower in some of the experi- 

 ments carried on in the season of 1904. However, further compari- 

 sons have failed to bear out this conclusion, and it is the belief of 

 the writers, based on careful observations on this subject, that the 

 inbred seed sprouts as rapidly as the cross-fertilized seed. It seems 

 probable that in the cases observed by growers in lOO-t a ditf'erenco 

 in moisture content of the rotted apple-tree fiber, the medium used 

 for sprouting, was the cause of the apparent differences in time of 

 sprouting. So far as the writers' observations go, the inbred seed 

 produces more rapidly growing plants than the open-fertilized seed, 

 and consequently earlier plants for transplanting. There is no doubt, 

 further, that the inbred seed produces a larger proportion of seed- 

 lings for transplanting at one time than the open-fertilized seed, 

 which is an important matter to the tobacco grower, who is fre- 

 quently forced to wait for seedlings on account of the lack of uni- 

 formity of plants in beds sown with open-fertilized seed. 



Darwin's conclusions on the comparison of tobacco plants raised 

 from inbred and cross-fertilized seed for three years are as follows: " 



Taking the plants of the three generations altogether, the crossed show no 

 superiority over the self-fertilized, and I can account for this fact only by sup- 

 posing that with this siiecies, which is perfectly self-fertile without insect aid, 

 most of the individuals are in the same condition as those of the same variety 

 of the common pea and of a few other exotic plants which have been self-fer- 

 tilized for many generations. In such cases a cross between tv\'o individuals 

 does no good ; nor does it in any case, unless the individuals differ in general 

 constitution, either from so-called spontaneous variation or from their pro- 

 genitors having been subjected to different conditions. I believe that this is 

 the true explanation in the present instance, because, as we shall immediately 

 see. the offspring of plants which did not profit at all by being crossed with a 

 plant of the same stock profited to an extraordinary degree by a cross with a 

 slightly different subvariety. 



a Darwin, Charles. Cross and Self Fertilization in the Vegetable Kingdom, 

 p. 210. 

 96 



