\ 



348 Harper : Native weeds and their probable origin 



stance of suitable material and form, such as a copper wire, is bent 

 slightly and released immediately it will quickly resume its original 

 shape ; but if bent too much or kept bent too long it will remain so.) 

 In New England and southern New York, to which the fol- 

 lowing discussion will be chiefly confined, the modification of plant 

 environments through human agency has come about chiefly in 



the following six ways. 



A. Trees of a forest destroyed by fire, lumbering, etc., and 

 allowed to grow up again without delay. Forest fires of course 

 occurred more or less before man existed, but their frequency has 

 been so greatly increased by civilization that they may now with- 

 out serious error be regarded as one of its effects. 



B. Trees removed and then kept down by repeated mowing, 

 burning, grazing, or trampling, as in hay-fields, railroad rights- 

 of-way, pastures, lawns, roads, and paths. 



C. Original vegetation all or nearly all destroyed and soil 

 disturbed, as in cultivated and abandoned fields, embankments, and 

 excavations. Similar conditions are sometimes, though rareiv, 

 brought about in a natural way by landslides, etc. 



D. Foreign matter added to the soil, as in gardens, barnyards, 



waste places, railroad yards, and ballast grounds. 



E. The amount of water changed, as in drained marshes an 

 swamps, irrigated lands, ditches, and artificial ponds. 



F. Entirely new substrata provided, such as roofs, wal s, 

 wharves, pavements, and refuse and artificial substances of a 

 kinds. 



t 



The plants which follow treatment A are practically all native 

 in the adjacent undisturbed forests or elsewhere in the vicinity, a 11 

 those of E are also usually native, within a few miles at leas . 

 After B and C, European weeds are numerous, if not in the ma 

 jority. The plants of D are nearly all exotics, and those of r ar 

 mostly cellular cryptogams of very wide distribution. 



* For the names of some of them see Meyen, Grundriss der Pflanzengeograp^ 

 87-92. 1836 (or pages 73-78 of the English translation, 1846). The al,ege as< , a 

 species of fungi which have been described in the last few years from such P 1 ^^^ 

 box where acetic acid had been spilled," and " among grasses where coal as e^ 

 been lying," as well as the numerous native fungous parasites of exotic plants, a 

 to be classed here. 



