Some native weeds and their probable origin 



Roland M. Harper 



Every botanist who attempts to classify the vegetation of a 

 populous region, such as the northeastern United States, is con- 

 fronted at the outset with the problem of distinguishing the natural 

 or undisturbed habitats from those which have been modified by 



civilization. Of course all our vegetation has felt the influence of 

 civilization more or less, but it seems possible to draw a fairly 

 sharp line between those habitats whose flora is essentially the 

 same now as it was in prehistoric times and those where it has 

 been so much altered that it is impossible to reconstruct the pri- 

 meval conditions. 



In general it seems to be true — and the task of the phyto- 

 geographer would be almost hopeless if it were otherwise — that 

 external influences of slight amount or of short duration produce 

 n ° permanent changes in vegetation. As an example of the first 

 ™d, when the pine trees are removed from an area of southern 

 Pine-barrens the amount of sunlight reaching the ground is 

 ^creased probably not more than 10 per cent., and this seems to 

 m ake no perceptible difference to the herbaceous vegetation.* But 

 « the ground is then plowed up and cultivated, the original vege- 

 tation disappears, most of it never to return. f 



In the second place, if a deciduous forest is destroyed by 

 umbermen or swept by fire it presents a very different appearance 

 or a time, but if left undisturbed it will regain its former appear- 

 anc e and flora, or very nearly so, as soon as the trees have time 

 grow up again. But if the cutting or burning is repeated every 

 CW years the ground will gradually become covered with herbs 

 and sh °rt-lived shrubs, among which it is difficult for trees to 

 re gain a foothold. 



(This tendency of vegetation to restore itself seems to be quite 

 ana 0gous to elasticity in physics. For example, if any solid sub- 



" * ee All »- nT^A^TscL i 7 : 1 17. 1906. 



T ^e Mohr, Contr. U. S. Nat. Herb. 6 : 120. 1901 



347 



