IOWA ACADEMY OP SCIENCES. 
103 
further evidence as to the degree of dissociation occurring 
there. 
The work recorded in the present paper was all completed 
previous to June, 1894, with the exception of the viscosity 
determinations. 
DISTRIBUTION OF SOME WEEDS IN THE UNITED 
STATES, ESPECIALLY IVA XANTHIIFOLIA, LAC- 
TUCA SCARIOLA, SOLANUM COROLINEUM 
AND SOLANUM ROSTRATUM. 
BY L. H. PAMMEL. 
One of the interesting parts dealing with geographical botany 
is the question of the distribution of plants over the earth’s 
surface, where man has played an important part. Many 
changes have occurred in the character of our North American 
flora since it has been occupied by man. In many cases it has 
become quite impossible to tell when and where plants were 
first introduced. We have, it is true, in some cases records 
when plants were introduced, but in the vast majority of 
cases there were no records at hand. Early collectors in 
many cases simply mentioned vague localities with dates, 
but say nothing as to whether the plants are indigenous or 
introduced. Papers dealing with the spread of certain weeds 
have been made by several investigators in both Europe and 
America. Franz Buchenau has carefully traced the spread of 
Leersia oryzoides. L. H. Dewey that of Salsola kali, var. Tragus."^ 
Who can attempt to trace the spread of Portulacca oleracea in 
the United States, or in any given state or territory, or such 
cosmopolitan weeds as Polygonum aviculare and Plantago major? 
No one has attempted to record the earliest appearance of these 
weeds in any part of the United States. 
The writer has for several years been interested in studying 
the distribution of several of our weeds. I shall attempt to 
* Russian thistle. Its history as a weed in the United States with an account of 
the means available for its eradication. Bulletin No. 15, Division of Botany, United 
States Department of Agriculture, p 26, Washington. 1894. 
