38 SOIL CONDITIONS AND PLANT GRO WTH 



In this case the falling off in activity at higher tempera- 

 tures (OE) is due to the destruction of the enzyme ; in the 

 case of the plant it is attributable to two factors, disadjust- 

 ment of processes and injury to protoplasm. 



(2) Variation of Two or more Factors. 



The simple case presented by single factor variation is 

 unusual in natural conditions : more generally two or more 

 factors are present in quantities insufficient for perfect growth. 

 This case has been discussed by F. F. Blackman who has in- 

 troduced the very happy phrase " limiting factor " to express 

 a conception previously used by H. T. Brown under the name 

 "throttle valve". Generally speaking the effect of each 

 separate factor is expressible by the single factor curve up to 

 the point where a second factor begins to be insufficient, and 

 then the curve alters considerably : instead of going on con- 

 tinuously the increase of growth falls off considerably or even 

 is brought to an end. A factor that thus proves insufficient 

 and stops or greatly retards what ought to be a continuous 

 process is called a " limiting factor ". Growth is once more 

 resumed when the amount of the limiting factor is increased 

 until again this factor proves insufficient, or some new factor 

 comes into play. 



These phenomena are illustrated by von Seelhorst's inves- 

 tigations on the effect of water supply on plant growth. In 

 one of their investigations Tucker and von Seelhorst (256) put 

 up three series of soil pots in which the water was kept at a 

 definite amount ; one was just moist, another was moister, and 

 a third still moister. These were then each subdivided into 

 three others, one receiving no nitrogen compounds, another 

 one dose, and the third two doses. Oats were sown in all 

 nine sets with results that are given in Table VI. 



When only little water is present the added 0-5 grm. of 

 nitrogen is without effect, the supply in the soil being suf- 

 ficient for the crop needs : the water and not the nitrogen is 

 the limiting factor. When more water is added the plant can 



