May 21, 1897.] 



SCIENCE. 



791 



in the number of species, of whicli there 

 are several, which have come from Europe 

 with comparatively clean passports to be- 

 come aggressive and troublesome in Amer- 

 ica. This is the case with the Hawkweed 

 {Hieraceum aurantiacum, Linn.), which is 

 just coming into prominence in some parts 

 of Canada and the ISTortheastern States.* 

 There are two reasons for this: First, the 

 comparatively very rapid changes, which 

 practically all America has undergone, 

 have largely destroyed the natural equilib- 

 rium of species, and this has made it 

 easier for capable weeds to creep in. Second, 

 the principle pointed out long ago by Darwin 

 as ' the good derived from slight changes in 

 the conditions of life,' applies to the case of 

 plants imported from Europe. This ' good ' 

 accrues to the species through induced 

 variability. 



Even more interesting points of inquiry 

 are revealed when we turn to study the 

 migrations of weeds within the United 

 States. Merely as a suggestion for further 

 work, I have made a few comparisons be- 

 tween Eastern weed floras and that of 

 Kansas. Professor A. S. Hitchcock f enu- 

 merates 209 species of Kansas weeds, of 

 which only 51 are foreigners against 158 

 natives. Even then nearly all the foreign 

 species are specifically stated to be rare in 

 the State. In my own list of the 20 worst 

 weeds of Kansas, instead of the remarkable 

 proportion of foreign species noted in Ver- 

 mont and !N"ew Jersey, there are 6 foreign 

 and 14 native species. It is also interesting 

 to note that exactly half this list is made 

 up of native composites. Of course, we 

 may expect that, as commerce goes on be- 

 tween Kansas and the Atlantic States, the 

 proportion of foreign weeds westward will 

 increase ; but we may feel confident that 

 the Daisy, the Hawkweed, and the Kales 

 will find no such easy time making head- 



* See Vermont Exp. Sta. Bull. 56 (1897). 

 t Kansas Exp. Sta., Bull. 57 (1896). 



way against the sunflowers and ragweeds 

 of Kansas as they have had against the 

 modest, shade-loving species of the Eastern 

 States. The native species of Kansas have 

 been used to live in the open country, ex- 

 posed to fire and drought and browsing 

 herds of buffalo. Now when they find a 

 well plowed field, with perhaps a little irri- 

 gation, they are fully prepared to occupy 

 the ground and hold their own against the 

 world. 



Then there is the question of these Western 

 species coming east. Sixty-five years ago 

 Rudheckia Jiirta was unknown east of the 

 Alleghenies, yet now it is widely distributed 

 in the Eastern States. Coreopsis tindoria, 

 Nutt., is a "Western Composite and a bad 

 weed, now much cultivated in gardens in 

 America and Europe. From these it has 

 already shown a tendency to escape, and 

 may be counted as a coming weed. Dysodia 

 chrysantliemoides, Lag., is said to be coming 

 rapidly eastward. Artemesia biennis, Willd., 

 also belongs to this list, and has recently 

 been collected in the railroad and dock 

 yards at Burlington, Vermont. This list 

 might be greatly extended. 



It seems probable that the great and 

 variable and geologically modern family 

 Compositse is destined to play an increas- 

 ingly important part in the future trans- 

 formations of American weed floras, and 

 that its representatives will be especially 

 prominent among the successful native 

 weed species, as, indeed, they already are in 

 the weed floras of the Mississippi Valley 

 States. 



F. A. Waugh. 



Univeesity of Veejiont. 



TBE EFFECT OF THE DENSITY OF TEE SUR- 

 ROUNDING GAS ON THE DISCHARGE OF 



ELECTRIFIED METALS BY X-RAYS. 

 It has been found that the rate of dis- 

 charge which occurs when X-rays strike 

 upon a charged body is affected by the 

 pressure of the gas surrounding such a 



