THE REQUIREMENTS OF PLANTS 



23 



insufficient and stops what ought to be a continuous process is called 

 by Blackman (40) a " limiting factor ". 



An instance of a limiting factor is afforded by Miss Matthaei's 

 work on the rate of assimilation in cherry laurel leaves. Working with 

 artificial light of low intensity she found that assimilation increased 

 with the temperature to a certain point, but then remained constant ; 

 the light was insufficient for quicker photosynthesis. When the light 

 was increased a higher speed of photosynthesis became possible, until 

 with full light the ordinary logarithmic curve was obtained. 



When all other factors are sufficiently supplied a limit is finally set 

 by the inability of the protoplasm to do more than a certain amount of 

 work. 



We can now draw up our general curve showing the relationship 

 between the supply of any particular factor and the amount of plant 

 growth. It consists of either two or three parts (Fig. 2). In the first 



Increment of Factor. 



Fig. 2. — General Relation between any particular Factor and Plant Growth. 



part all the processes in the plant are working harmoniously and the 

 plant remains healthy ; here an increase in the factor causes an increase 

 in the amount of growth, and the curve is similar to that obtained 

 for any single process considered separately. In the second part some 

 limiting factor comes into play, such as an insufficiency of something 

 essential, or an inability of some process to go any faster ; the rate of 

 growth cannot, therefore, show any further increase. It may happen 

 that further increase of the factor even acts injuriously by bringing 

 about a secondary adverse effect such as injury to the protoplasm or 

 to the medium in which the plant is growing. 



Mitscherlich has shown (201 and 202) that in some cases, where 

 the adverse effect is absent, the curves can be expressed by a simple 

 equation. If all the conditions were ideal a certain maximum yield 

 would be obtained, but in so far as any essential factor is deficient, 

 there is a corresponding shortage in the yield. The yield rises if some 



