THE CAUSAL PAEASITE. 25 



spread the trouble, and in some cases the manner of introduction 

 can not be determined. 



Close analysis of all the earlier reports and of the existing distribu- 

 tion of root-knot has convinced the writer that we have to deal \vith 

 a pest originally tropical or subtropical in its distribution and not 

 native to any part of the United States. In this the writer comes 

 to a conclusion at variance with tliat of Neal/ who beheved that it 

 was native to the Southern States. If that were the case, however, 

 it ought to be found on uncleared land where no crops have ever 

 been grown, but that is not generally the case. Indeed, it is the 

 general practice, when nematode-free land is needed, to go to un- 

 cleared land. To be sure, nematodes are occasionally found in such 

 land, but almost always it can be shown to have been previously in 

 cultivation, perhaps many years ago, or to be so situated that soil 

 from infested fields could be washed upon it. 



The general trade in exotic plants which began over a hundred 

 years ago and grew rapidly, in the course of which ornamental and 

 usefid plants from the Tropics, especially of the Americas, were car- 

 ried to European conservatories and gardens and also to our shores, 

 may very probably have served to introduce the pest into the tem- 

 perate regions of both the Old World and the New World. In all like- 

 lihood the Spaniards introduced this nematode into Florida directly 

 from the West Indies or Central America, for it is found in parts of 

 southern Florida that were in cultivation more than 75 years ago, 

 but where now dense forests have grown up, as well as in clearings 

 with no signs of recent cultivation. Yet even here it does not seem 

 to occur in land absolutely unused in the past. 



Wliether the Old World or New World Tropics were the original 

 home can not be decided now, as it is widely distributed in both. 

 Perhaps its wide distribution in Africa, India, the East Indies, 

 China, and Japan and the fact that another species of the same genus 

 (Heterodera scJiachtii Schmidt) is apparently native in Europe would 

 warrant the conclusion that it is probably of Old World origin. 



THE CAUSAL PARASITE. 



Upon breaking across a medium-sized or large knot and holding 

 the broken surface so as to reflect the light a close observer will often 

 see one to many clear to ahnost pearly white rounded bodies, con- 

 siderably smaller than half the diameter of a pinhead, projecting 

 from the surface. With a hand lens they are easily seen, but for the 

 unaided eye they are sometimes verj'- difficult to detect, on account 

 both of their minuteness and of their transparency. In knots 

 that have been cut across they are usually not visible, as they col- 



1 Neal, 1889. 

 217 



