472 PRACTICAL BOTANY 



agricultural experiment station there ^^■ere found in all 87 

 species of other seeds, mostly those of noxious weeds, belong- 

 ing to 23 different families of plants.^ 



435. Other injuries caused by weeds. Aside from the dam- 

 age inflicted by weeds upon growing crops and farm animals, 

 much harm is done by them in less obvious ways. 



Roadside weeds of many species encroach upon roads of all 

 kinds, from country byways to, city boulevards. Among the 

 weeds of waste ground there are many which disfigure the 

 surface of vacant city lots, and the numerous burs among 

 them load the passer with their clinging seeds or fruits. 



Railway rights of \^'ay, if left uncared for, soon become over- 

 grown with weeds, which shade the ties and cause them to 

 decay more rapidly. It is estimated that the expense of re- 

 moving weeds from the railway tracks in the state of Ohio 

 alone exceeds |500,000 a year. 



Streams, canals, and drainage or irrigation ditches are often 

 infested by weeds, which may almost stop the current of water 

 in them. The water weed, or ditch moss (Elodea), introduced 

 into Europe from America, has become a nuisance there, choking 

 small streams with its abundant growth. The so-called water 

 hyacinth (^Eichhornici) from South America, often cultivated 

 in aquaria and small ponds, has been introduced into Florida 

 and other southern waters, where it greatly impedes navigation. 



436. The origin and dissemination of weeds. One of the 

 interesting facts Math which the young botanist is first im- 

 pressed on beginning to identify weeds and to trace their his- 

 tory, is the extent to which they ha\'e immigrated from other 

 countries.^ No one can calculate with exactness the propor- 

 tion of our weeds (that is, of individuals) which liave been 

 brought in from other countries. But it is not difficult to see 

 how the numbers stand in comparing natiAe and introduced 



1 See Bulletin 175, Ohio Agr. Exp. Sta. 



^ See the article, "Pertinacity and Predominance of Weeds," in tlie 

 Scientific Papers of Asa Gray, selected by C. S. Sargent, Vol. II, Houghton 

 MifiSin Company, Boston ; also Farm Weeds of Canada, Second Edition, 

 Government Printing Bureau, Ottawa, Canada. 



