1909 Adams. — JDtstinguishing Plants m h eland. 127 



It remains to be seen next whether any of the doubtful 

 species (Group C) have sufficiently strong claims to justify 

 their inclusion among the genuine natives of the country. 

 We have, as it were, to examine their credentials. Are there 

 any tests which can be applied to settle the question whether 

 such and such a species is a native or an alien in this country ? 

 Doubtless there will still remain some species as to whose 

 exact status there will never be any consensus of opinion, but 

 it ought to be possible to determine, with a reasonable amount 

 of certainty, which of the " suspects " are natives and which 

 are aliens. 



Before dealing with the Flowering Plants it will be advis- 

 able to refer briefly to the groups of Cryptogams which have 

 been introduced. The majority of these belong to the great 

 group of Fungi. In the case of a parasitic fungus, if the host 

 plant on which it occurs be an introduced species, the fungus 

 will then be considered an alien also. As an example, the 

 common Potato Blight {Phytophthora infestans, De Bary) will 

 be considered an alien until it is found growing in this 

 country on Solanum Dulca^naj'a^ I^inn., when its claim to be 

 native may possibly be conceded. The same test will apply to 

 saprophytes. If the medium on which the fungus grows is a 

 foreign substance, such as the wood of Laburnum, or rotting 

 Oranges or Bananas, and, if it is not found growing on any 

 other medium, then it may fairly be regarded as an alien 

 whose spores were introduced with the decaying vegetation on 

 which it grows. 



Among higher Cryptogams a few species of Ferns have been 

 found on walls to which their spores were, doubtless, blown 

 from the nearest garden. 



Returning to the higher plants, or Phanerogams, the fol- 

 lowing questions are those which suggest themselves as likely 

 to help in solving the question of the nativity or alien nature 

 of any doubtful species. 



I. What is the habitat of the species hi question^ and how does 

 this compare with its habitat in countries in which it is known to 

 be native ? House-Leek, for instance, is found in this country 

 only on walls and roofs ; on the Continent it occupies similar 



