WHAT PLANTS NEED 307 



pursue is not to apply some treatment that is claimed to be a 

 cure-all, but to discover, if possible, the limiting factor which 

 is holding back the usefulness of the numerous factors that 

 are all right. A plant that is not doing its best is like a 

 train moving with brakes set ; and the thing to discover is the 

 factor that is acting as a brake. 



The increased success of agriculture and of horticulture 

 will depend largely upon the development of an ability to 

 recognize limiting factors. The difference between the old 

 agriculture and the new will be the difference between the 

 method of the old medical practitioner, with his calomel and 

 quinine for every ailment, and the method of the modern 

 practitioner, with his developed powers of diagnosis and his 

 great variety of prescriptions. It is very important for the 

 cultivator of plants, at the very outset of his training, to pos- 

 the idea of limiting factors. 



