350 Harper: Native weeds and their probable origin 



Blankinship's list is rather fragmentary, Dr. Britton's includes a 

 number of introduced species, and neither is as sharply limited as 

 it might have been. The flora of the Hempstead Plains, which is 

 somewhat similar, has never been published, but it seems to com- 

 prise about 80 native species of vascular plants. 



4. Species undoubtedly native in open woods, pine-barrens or 

 prairies farther south and west, which have probably come into the 

 northeastern states only since the clearing away of most of the 

 forests has destroyed much of the humus and made the summers 

 warmer. To this class probably belong a number of species 

 Compositae, Leguminosae, Crataegus, etc., but it would be dim- 

 cult to specify any particular species. 



5. Species which grew in Indian clearings before the white man 

 came, so that it is now impossible to determine where they are 

 really native. There is not much on record about these in the 

 parts of our country which have been settled the longest. Phyto- 

 lacca decandra is supposed to be one of them, and a few others of 

 southern distribution are mentioned in Mohr's Plant Life of Ala- 

 bama, page 54. Such plants are mostly to be looked for in 

 first three classes of unnatural habitats, and they cannot be very 



the 



numerous. 



6. Species not included in any of the five foregoing classes, 



ently 



supposed to be native in the eastern United States, but appai 

 confined to unnatural habitats, and behaving like introduced plants, 

 from which they are to be distinguished in most cases only by docu- 

 mentary evidence. These will be discussed at length farther on. 



Weeds 



all of them have been accustomed to unnatural habitats for cen- 



I 



turies. 



D 



of natives, are very common in C y less so in B, rare in A, and are 

 wanting in perfectly natural habitats. The number of weeds cer 

 tainly known to have been introduced into this country is q" lte 

 large, constituting at present in the northeastern states about 20 

 per cent, of the angiospermous flora. " 



8. Exotic plants which grow spontaneously for a time, but art 



*For an interesting discussion of some plants of this and the next class see 



Fern 



aid's lecture on "Some recently introduced weeds" (Trans. Mass. Hort. Soc. I9°5 



11-22. 1905). 



