714 WEED FLORA OP IOWA 



caria discoidea of Asia, Mexican fireweed ( Kochia scoparia), brome 

 grass (Bromns tectorum) in the vicinity of Bremen; all being in- 

 digenous to eastern Europe and western Asia. 



Dr. Fernald states that of the species of plants growing in the 

 British Isles, at an altitude of 3,000 feet in the mountain country, 

 where the primitive vegetation is but little disturbed, 68% are also 

 native to the mountainous regions of New England, but of the 

 species that occur everywhere at low altitudes and in thickly set- 

 tled and closely farmed districts only 23% are native to New Eng- 

 land while more than 50% have become established in New England 

 as weeds; and that barely 1% of the plants of temperate Europe 

 have been imported from America, but every year adds to the new 

 weeds introduced into Europe. 



Dr. Gray, in that interesting paper on the Pertinacity of Weeds, 

 refers to the book by Mr. Henslow on the Self-Fertilization of 

 Plants, in which the latter comes to the conclusion that weeds are 

 intrusive and dominating plants of great emigrating capabilities, 

 and have a longer ancestral life history than their more or less 

 aggressive relatives. Furthermore, this dominance may be at- 

 tributed to self-fertilization. In the list of weeds that Dr. Gray 

 mentions in this paper as the most abundant in eastern North 

 America and the southern United States, the rule evidently does 

 not hold that this dominance is due to self-fertilization; in fact 

 some of the most dominant of these plants are pollinated by in- 

 sects. For the sake of convenience I shall here use a list prepared 

 by Dr. Gray in 1879, adding to it weeds that are more or less pre- 

 dominant in the state of Iowa at the present time ; the Iowa weeds 

 are printed in small capitals. There is a slight rearrangement, 

 according to present interpretation of taxonomists. 



Gramineae. 



Digitaria humifusa, Smooth crab grass. 



D. sanguinalis, Crab grass. 



Panicum capillare, Old-witch grass. 



P. dichotomiflorum, Sprouting crab grass. 



Echinochloa crusgalli, Barnyard grass. 



Setaria glauca, Pigeon grass. 



S. viridis, Green foxtail. 



S. verticillata, Bristly foxtail. 



Cenchrus tribuloides, Sandbur. 



Anthoxanthum odoratum, Sweet vernal grass. 



