702 WEED FLORA OF IOWA 



Medicago denticulata, and Foenicidum capillaceum. In the flora 

 of Buenos Ayres, according to Otto Kunze, at least three-quarters 

 of the species are introduced, and largely Mediterranean in source. 



Roland M. Harper says: 



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 primeval 

 conditions. 



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

 geographer Avould be almost hopeless if it were otherwise — that 

 external influences of slight amount or of short duration produce 

 no permanent changes in vegetation. As an example of the first 

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

 pine-barrens the amount of sunlight reaching the ground is in- 

 creased probably not more than ten per cent, and this seems to 

 make no perceptible difference to the herbaceous vegetation. But 

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

 tation disappears, most of it never to return. 



In the second place if a deciduous forest is destroyed by lumber- 

 men or swept by fire it presents a very different appearance for a 

 time, but if left undisturbed it will regain its former appearance 

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

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

 few years the ground will gradually become covered with herbs 

 and short-lived shrubs, among which it is difficult for trees to 

 regain a foothold. 



M. L. Fernald has said along the same line : 



The clearing of the forest lands and the letting in of the direct 

 sunlight is the inevitable forerunner of the farm and the village, 

 but it is as inevitably the death warrant of hundreds of native 

 plants. As is now well understood, a majority of our woodland 

 species have a root structure which allows them to grow only in 

 the moist, spongy humus of the forest or the swamp, conditions 

 as many of us know from practical experience, almost impossible 

 of artificial attainment. Try as we will, most if not all of us 

 have failed to imitate with sufficient skill permanently to satisfy 

 the plant the exact conditions which please the stemless lady's 

 slipper (Cypripedium accmle), the trailing arbutus (Epigaea), the 

 various species of Pyrola, the yellow wild foxgloves ( Gerardia) , the 

 painted-cups (Castilleja) , or the fringed gentian; though in their 

 undisturbed haunts these plants bloom regularly and reproduce 

 freely. 



