80 THE POPULAR SCIENCE MONTHLY. 



nal defeat that native plants often incur at the hands of invading 

 strangers. 



Why does the water-cress, harmless enough in our ditches, block 

 up the water-courses in New Zealand to such an extent as to become 

 a costly nuisance ? What can there be in English ditches and canals 

 so propitious to the growth of the American water-weed (Anacharis) 

 as to have caused it to obstruct even our navigable rivers ? In Amer- 

 ica, whence it came, it is no more of an inconvenience than any other 

 water-weed. Why in other places does the white clover (Trifolium 

 re})ens) overcome the native grasses, and dispossess them of their ter- 

 ritory ? Why has a particular grass, the Stipa tortilus, invaded the 

 South-Russian steppes to such an extent as to displace almost every 

 other plant ? 



There are numberless such instances from that afforded by the 

 island of St. Helena, in which the original vegetation is almost com- 

 pletely dispossessed, and its room occupied by foreign importations, 

 to the banks of a Surrey river, yellow with the flowers of an American 

 balsam and the reason is not obvious. The fact is patent, and is not 

 without analogies in the virulence with which epidemic diseases spread 

 when introduced for the first time among a population not heretofore 

 subjected to them. 



Such cases as these recall the opinions of Humboldt and others on 

 the antipathies of plants. According to this notion, certain plants are 

 positively injurious to others, not so much by any peculiarity of struct- 

 ural organization as by the excretion of matters hurtful to other 

 plants. It has been asserted, for instance, that the darnel (Lolium 

 temulentum) is injurious to wheat; that a species of thistle (Serratula 

 arvensis) is obnoxious to oats ; that a spurge (Euphorbia Peplus) and 

 a scabious (Knautia arvensis) are detrimental to flax ; and spurrey 

 (Spergula arvensis) similarly prejudicial to buckwheat. 



In so far as this detrimental influence is due to any excrementitious 

 product from the plant, the verdict given by modern physiologists 

 amounts to " not proven." Some would even say " not guilty ; " but 

 we do not see clearly how those who take this view can reconcile 

 it entirely with the existence of that natural alteration of which 

 Dureau de la Malle speaks, and which is admitted by all subsequent 

 observers. 



Mere exhaustion of the soil Will not account for the phenomena in 

 all cases, because a crop will fail on a particular soil after a while, and 

 yet chemical analysis of that soil will reveal the fact that the particu- 

 lar elements required by a given plant are still contained in sufficient 

 abundance in it. Land, for instance, that is " clover-sick " on which, 

 that is, good crops of clover cannot be grown is by no means neces- 

 sarily deficient in the constituent required for the growth of the plant ; 

 and, indeed, in the Rothamsted experiments the constituents in ques- 

 tion have been supplied as manure, but without any good result. 



