No. 565] CHANGES PRODUCED BY SELECTION 



47 



practical importance of determining the duration of the 

 period in the course of which particular plant characters 

 are responsive to the action of environmental 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 practise. 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. 2 But seed selected from mother 

 plants grown on the good soil will produce plants aver- 

 aging slightly higher in leaf number than the plants com- 

 ing from seed on mother plants whose environment is 

 poor. Consequently, it is better to select seed from well- 

 developed mother plants — mother plants whose environ- 

 ment 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 in- 

 fluenced by external conditions within the limit of fluctu- 

 ating variability. 



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 transplanta- 

 tion is negligible and the plant continues its normal cycle 

 of development without a pause, bringing to maturity 



