122 POPULAB EEKOES. 



crop rotation, or the introduction of plants of a 

 different kind, lay in the utilization of these 

 excreta, or at least in allowing time for them to 

 disappear from the soil before plants of the origi- 

 nal kind were again grown. The more the subject 

 of crop rotation was studied, however, the more 

 dif&cult it became to reconcile the observed facts to 

 the excretory theory. It was found that on some 

 soils the rotation of crops was unnecessary, but 

 that the same crop might be grown year after year 

 indefinitely. Other causes began to be discovered 

 to aacount for the advantages derived by rotation of 

 crops. Finally, Alfred Gyde, of Scotland, and 

 others, re-examined the subject of root excretion 

 and proved that very little if any actual excretion 

 takes place, and that what appeared to be such in 

 fdrmer experiments consisted of the ordinary juices 

 of the plant which had exuded from the broken roots 

 of the plants employed in the experiments. They 

 also proved that even this material was beneficial 

 rather than otherwise when applied to the roots of 

 other plants of the same kind. 



These experiments were so conclusive and satis- 

 factory that it is now generally accepted that no 

 proper excretion from the roots of plants takes 

 place. 



Further experiments have shown, however, that 

 many, if not all, plants exude an acid or alkaline 



