No. 565] CHANGES PRODUCED BY SELECTION 35 



resulted in higher means, those for F 7 , F 8 and F 9 being 

 25.3 ± .09, 26.0 ± .06 and 25.9 ± .08 leaves, respectively. 



This peculiar result implies only that the mean of the 

 original F 6 population which was grown at Bloomfield 

 was lower than it would have been if grown on the Forest 

 Hills' soil. This is not a direct effect of environment on 

 the growing plant. It has been shown conclusively in 

 our pot experiments, as stated before, that starvation or 

 optimum feeding has scarcely any effect on the number of 

 leaves, although it has a marked effect on the develop- 

 ment of many other characters. On the other hand, en- 

 vironment does appear to have a marked effect on the 

 number of leaves that a plant is to develop, if it acts 

 during the development of the seed. It is well known by 

 plant physiologists that the environment produces many 

 of its effects very early in the life history of the indi- 

 vidual or in the development of the organ concerned. For 

 example, the so-called light leaves of the beech with two 

 layers of palisade cells are differentiated from the shade 

 leaves with only one row of palisade cells by the amount 

 of light that falls on a branch during the season preceding 

 the development of the leaves: that is, it is determined 

 during the laying down of the bud from which the next 

 season's growth of twig and leaves comes. This period 

 during which a particular change is possible is called the 

 critical period for that change by plant physiologists. 

 Thus a plant may have hundreds of critical periods in its 

 ontogeny, each marking an end-point of development be- 

 yond which a certain feature is irrevocably fixed. For 

 example, the critical period for that cell division that de- 

 termines leaf size in the beech is much later than that 

 which determines the number of layers of palisade cells. 



Now the critical period for influencing the number of 

 leaves of the tobacco plant is practically at an end when 

 the embryo plant goes into the resting stage of the seed. 

 Before that time the number of leaves may be influenced 

 by the external and the internal influences that form the 

 total environment of the mother plant; after that time 

 environment has little influence on the number of leaves. 



