CH. i] Overstepjnng the Limits 7 



make any more growth no matter how much food or 

 water is given. Indeed it is easy to overstep the Umit 

 and give too much so that the crop actually suffers. 

 This has happened in the experiment recorded in Fig. 4. 

 Here, as in Fig. 1, tomatoes are shown growing in soils 

 provided with different amounts of manure. The first 

 and second doses of manure resulted in an increased 

 crop: the third dose caused no further increase: while 

 the fourth actually caused a decrease, the excess of food 

 now acting as an injurious substance. This is well seen 

 also in Pots 27 and 36, Fig. 5 (top row). 



The hmit reached in any particular instance, however, 

 is not necessarily the best growth that can be obtained. 

 It ma.y be set by the insufficiency of water, of tempera- 

 ture, etc. Fig. 5 shows in the upper part a set of tomato 

 plants supplied with successively increasing amounts 

 of manure and 5 per cent, of water; in the middle a set 

 supphed with the same amounts of manure and 10 per 

 cent, of water; and in the lower part a third set also 

 receiving the same quantities of manure but 12-5 per 

 cent, of water this being as much as the soil would 

 hold. The hmit of growth reached in the first case is 

 clearly due to a deficiency of water, for it is raised con- 

 siderablv when more water is added. But a still further 

 increase in the supply of water does not lead to more 

 growth, the limit being now set by something else. It 

 is possible that by increasing the temperature or the 

 root room we could get more growth out of this last 

 series, but the process comes to an end before long and 

 the final limit is set by the sheer inability of the plant 

 to grow any bigger. If larger crops are wanted it be- 

 comes necessarj' to try some bigger yielding variety, i.e. 

 some plant with more power of growth. 



