86 The Irish Naturalist. June, 



NOTES ON SOME ALIEN PLANTS OF 

 COUNTY DUBLIN. 



BY NATHANIEL COLGAN, M.R.I. A. 



The discrimination of native from introduced species is one 

 of the most difficult of the problems which confront the 

 compilers of local floras, and this difficulty is to a great 

 extent insurmountable, since it arises mainly from the 

 defect of early records for alien plants. If systematic 

 botany were not what it is, a creation of yesterda^^ if some 

 tenth century Linnaeus had arisen to lay thus early the 

 foundations of the science and render possible the pro- 

 duction on modern lines of a twelfth century Flora of 

 Co. Dublin, how profoundly would our views as to the 

 standing of certain species be altered. How conspicuous 

 would be the absence fromi such an early Flora of a host of 

 denizens and colonists now fully domiciled in the country, 

 what a striking poverty of species we should find, and how 

 convincing a proof we should have of the far-reaching 

 effects of unconscious human agency in the enrichment 

 of a fiora. If we regret the want of such early historical 

 records as would give precision and certainty to our judg- 

 ments on the standing of doubtfully native species that is 

 all the more reason why we should endeavour to supply 

 the future local botanist with trustworthy materials. It 

 is not enough to note the first appearance of immigrant 

 species. Their var3dng fortunes should be followed up by 

 continuous observation, and the object of these notes is 

 mainly to record the result of such observation applied to 

 a few aliens whose first appearance or apparent establish- 

 ment was recorded in the " Flora of Co. Dublin " fully 

 thirteen years ago. The numbers prefixed to records here 

 refer to the botanical districts as set out in the Flora. 



Sisymbrium pannonicum, Jacq. 



5. Ilalf-a-dozen plants at Howth railway station, 19 15. 7. Two 

 plants at Ballsbridge, 191 7. 8. Several plants in Pembroke Park, 

 1907 ; a few plants on the railway line at Dalkey station, one at Killiney 

 station, fully 150 on the abandoned railway line south of KiUiney, and 



