Varietäten, Descendenz, Hybriden. 7 



results. It is important to knovv whether plant or animal populations 

 can reach such a State of constancy by inbreeding that no profitable 

 results can afterwards be obtained by the practical breeder. We 

 believe it demonstrated by even these few data that such a State, 

 a homozygous condition, occurs in a definite proportion of Fg 

 offspring, and can be propagated commercially at once if a sufficient 

 number of families are grown to be relatively certain of including 

 the desired combination. 



The writers add to their conclusions some general considerations 

 about the question of the true constancy of homozygotes generation 

 after generation, and about the occurrence, possibility and cause 

 of mutation. They do not consider „mutation" as of economic value 

 but believe that the isolation of homozygous strains from mixtures 

 that are either mechanical or physiological, that are either made 

 artificially or are found in nature, offers the only method of procedura 

 that the practical breeder Avill lind financially profitable. 



Finally the writers call attention to the practical importance of 

 determining the duration of the period in the course of which 

 particular plant characters are responsive to the action of environ- 

 mental influences. The character complex that has been the basis 

 of this study is a striking illustration of how results from such 

 investigations may be applicable to farm practice. One may plant 

 a Portion of the seed from a self-pollinated tobacco plant on poor 

 soil or on good soil and the average number of leaves per plant 

 and the general Variation of the plants in number of leaves will 

 remain nearly the same in both cases. But seed selected from 

 mother plants grown on the good soil will produce plants averaging 

 slightly higher in leaf number than the plants coming from seed 

 on mother plants whose environment is poor. Consequently, it is 

 better to select seed from well developed mother plants — mother 

 plants whose environm.ent has been good — than from mediocre 

 mother plants. There is no question here of the inheritance of an 

 acquired character or of continuing to raise the number of leaves 

 by cultural treatment. One simply takes advantage of the fact that 

 during seed formation there is a period of mobility at which time 

 the Potential number of leaves of the young plant are practically 

 fixed. Pending the end of this critical period, the number of leaves 

 can be infiuenced by external conditions within the limit of fluctuating 

 variabilit3^ 



In the same connection, the effect of time of planting on the 

 tobacco plant should again be mentioned, as this also emanates 

 from environmental change. The actual number of leaves is, of 

 course, practically fixed at the time of setting the plants in the 

 field, but this is not true of the number of leaves that will have 

 a commercial value. For example, a seedling with 26 potential 

 leaves is planted. If it is planted when about four inches high, the 

 general physiological disturbance due to transplantation is negligible 

 and the plant continues its normal cycle of development without a 

 pause, bringing to maturity about 22 leaves. If planting is delayed 

 until the seedling is eight or ten inches high, there is a different 

 State of affairs. Development is arrested, the plant pauses to adjust 

 itself to the change. It soon recovers and continues its normal 

 ontogeny, but the period of reduced growth has left an ineffaceable 

 record. Several of the leaves — among them the more valuable 

 leaves — have been so affected during this readjustment, that they 

 develop to only a fraction the size that they should attain because 



