THE AMERICAN BOTANIST. 93 



in the soil something which prevents the roots of similar plants 

 from growing there. By the addition of fertilizers succeeding 

 crops are able to rid themselves of these harmful substances 

 and here is found the reason for fertilizing the land. A rota- 

 tion of crops contributed to the same end by allowing time, 

 between two crops of the same kind, for the harmful substances 

 to disappear. 



Acclimatization. — For every plant there is a certain 

 maximum, minimum and optimum temperature which de- 

 termines its life processes. These processes go on best at 

 some point between the maximum and minimum temperatures 

 called the optimum, but are active in lessened degree on both 

 sides of the optimum until the minimum or cold limit or the 

 maximum or heat limit is reached. Beyond these, the plant 

 either dies or goes into a resting condition until the return 

 of better conditions. The points at which heat and cold stop 

 the activities of plants are often called the cardinal tempera- 

 ture points. Frequently a plant will have one set of cardinal 

 points for growth and another for seed bearing. Acclimatiza- 

 tion of a plant consists in removing it to a warmer or colder 

 region and by cultivation or selection, raising or lowering its 

 cardinal points to conform to that region. Since, as we have 

 mentioned, plants may have different cardinal points for differ- 

 ent functions we may acclimatize one and not the other. Many 

 plants of warm regions when removed to colder ones will grow 

 but are unable to lower their cardinal points for fruiting and 

 thus do not flower or make seeds. On the other hand, arctic 

 plants will often grow luxuriantly in warmer climates, but fail 

 to fruit. Only those plants that are capable of raising or 

 lowering their cardinal points to a great extent are able to 

 make any great north and south extension of their range. 



