10 38 



Proceedings of the Royal Irish Academy. 



express their past history. The test which we apply to them is of 

 necessity a three-fold one — source, dispersal, habitat. Dunn's definition, 

 already quoted, of a native plant — a plant which has spread by natural means 

 from a natural source to a natural habitat — is the only possible one, if we 

 follow the matter to a logical conclusion. Now, a plant may fail to fulfil 

 any one, any two, or all three of these conditions — its source, its dispersal, 

 its habitat, may be any or all under suspicion. Eight combinations of these 

 three conditions are possible, and any of these eight combinations may occur 

 in nature, though some occur much more frequently than others. "We may 

 tabulate these possibilities ; and if we let 1ST stand for uncontaminated condi- 

 tions, and * for contaminated conditions, then we can express the standing of 

 a plant as shown in the right-hand column : — 



Source. 



Dispersal. 



Habitat. 



Symbol. 



Natural 



Natural 



Natural 



NNN 



Natural 



Natural 



Artificial 



NN 4 



Natural 



Artificial 



Natural 



N * N 



Natural 



Artificial 



Artificial 



N * * 



Artificial 



Natural 



Natural 



* NN 



Artificial 



Natural 



Artificial 



* N * 



Artificial 



Artificial 



Natural 



* * N 



Artificial 



Artificial 



Artificial 



* * # 



The reader will easily select instances from his own experience in 

 which every one of these eight sets of conditions may occur or has 

 occurred. I hesitate to apply any of the existing terms for the standing of a 

 plant, such as " colonist," " denizen," &c, to any of these combinations, since 

 these have been used rather vaguely, and not always in the same sense ; 

 neither do I suggest names for them, since the formula itself is shorter than 

 any descriptive name would be, and definite and self-explanatory besides. 



"We need to be clear about our definition of each of the three tests. 

 A native source means one which has never been contaminated, thus carrying 

 us back to the pre-human period ; this category deals especially with the various 

 habitats which the plants' progenitors may have occupied before arrival in the 

 present habitat. Natural dispersal in the same way must of necessity refer to 

 the continued dispersal of the progenitors of our chosen individual from the 

 beginning, and this category deals with the means by which the said progenitors 

 migrated from habitat to habitat until the present station was reached. The 



