IN THE VEGETABLE KINGDOM 45 



without straw. We have seen that the maximum degree 

 of fertility of which the plant is capable will be decided 

 by inheritance. Clearly, then, from absolute barrenness 

 the degree of fertility will develop in a direct ratio with 

 the supply of nutriment until the inherited potential 

 maximum is produced. It is then, and only then, that 

 the above rule will become operative. The full principle, 

 then, may be set forth thus : 



The variation of the degree of vegetable fertility in 

 response to the direct action of the environment will 

 be directly proportional to the development of vitality 

 (or protoplasmic energy) until the optimum point for 

 fertility is reached ; it will thenceforward be inversely 

 proportional to the development of vitality. 



Thus this generalisation really represents the applica- 

 tion of the principle of the vital optimum to the question 

 of fertility. For the organism there is always a best 

 point, degree, or quantity of every factor in the environ- 

 ment. There is always a best temperature for the 

 organism, a best degree of moisture, a best quantity of 

 any nutritive factor. This best point or degree is not 

 always the same, even for the same organism, but is 

 subject to variation. Yet, no matter what may be the 

 optimum point at any given time, any variation of con- 

 ditions away from that optimum is to that extent harmful 

 to the organism. 



Obviously there must also be an optimum or best 

 point for fertility for any factor in the environment, 

 and the arrangement set forth in the generalisation given 

 above seems to be the only one possible. The organism 

 must be able to adjust its rate of increase, not only to 

 conditions to which it is well adapted, but to conditions 

 of extreme richness and extreme poverty. As the maxi- 

 mum potential degree of fertility will be decided by 



