502 PRACTICAL BOTANY 



numerous suckers from the roots, quickly gets a footing and 

 holds its ground to the exclusion of most other vegetation. 

 In 1855 only 44 species of foreign plants had become natural- 

 ized in New Zealand, while in 1895 the number had increased 

 to 500 or more. Evidently the newcomers have qualifications 

 which enable them to succeed better than many native plants. 1 



Botanists are coming to recognize more clearly than ever 

 before that multitudes of plants grow under conditions which 

 are really unfavorable for them. It is thought that they do 

 so, occurring in abundance in uncongenial stations and infre- 

 quently amid better surroundings, because the unfavorable 

 conditions are so severe as largely to exclude the competition 

 of other plants. For example, knotgrass, 2 knawel, 3 milk purs- 

 lane, 4 and cudweed 5 do not grow in much-trodden paths be- 

 cause they are benefited by being trampled and by having to 

 root themselves in hard soil, but because in such paths they 

 are not crowded and so overshadowed by taller weeds. Seed 

 plants which usually grow on nearly bare rocks, as on cliff 

 sides, generally flourish better (with an equally favorable 

 light supply) in richer and moister soil. Desert plants, as 

 many cacti, often grow more luxuriantly under conditions of 

 soil and climate such as suit ordinary mesophytes. The com- 

 mon groundsel, 6 which abounds on the clean sand of some 

 Mediterranean beaches, blossoming and seeding with an un- 

 branched stem only about an inch high, in fertile, fairly moist 

 soil may grow to a height of 18 inches. 



It is not safe to assume of any species that the territory or 

 station in which it most commonly grows is the best adapted to 

 its needs. Such a statement could only be made with assurance 



1 On the subject of the spread of species introduced into new territory 

 see Darwin, Origin of Species, chap, iii ; Gray, essay on "The Pertinacity 

 and Predominance of Weeds," in Scientific Papers of Asa Gray, edited by 

 Sargent (Houghton Mifnin Company, Boston, 1889) ; also weed reports of 

 the state agricultural experiment stations and of the United States Depart- 

 ment of Agriculture. 



2 Polygonum aviculare. 8 Sd&ranthus annuus, 4 Euphorbia maculata. 



5 GnapJmlium uliginoswn. 6 Senecio vulgaris. 



