xxxvi INTRODUCTION 



a permanent lodgment in the country, others, no doubt, still depend 

 for their continued existence on the periodical introduction of fresh 

 seed, and the protection against aggressive native species, which 

 is afforded by the operations of husbandry. The number of 

 colonists, denizens, and casuals rapidly decreases as we pass from 

 light cultivated soils, to areas where the primitive surface is 

 still undisturbed ; the bogs, mountain-sides, and limestone crags 

 generally possess a strictly indigenous flora. 



On the other hand, we may over-reach the truth if we deny a 

 plant's nativity because it is now often found in association with 

 the works of man. Just as built walls are now the favourite 

 haunt of many native plants which must liave been originally 

 confined to limestone rocks or such places (e.g. Ceterach officinarum, 

 Asplenium Ruta-miiraria, Parietaria officinalis), so no doubt some 

 of the plants of cultivated fields and other artificially prepared 

 habitats are truly indigenous species, which willingly forsake their 

 limited native haunts for the wider field offered to them by human 

 operations. 



A large number of the plants of cultivated ground favour light 

 soils. They are rarer in the west than in the east of Ireland, and 

 reach their maximum in the south-east, where soil and climate 

 best suit their requirements. 



From the plants which have spread from gardens, or were sown 

 with crops, we may turn to others of purely casual introduction. 

 The majority of these are of sporadic occurrence, and frequently 

 their unfamiHar forms appear for only a single season ; but certain 

 others have become thoroughly naturalized, and have spread far 

 and wide over the country. The centres of dispersal of this element 

 of the flora lie in the towns and harbours, where, about quays 

 and railways, and waste ground, they maintain a precarious 

 existence. Of those which have obtained a footing, certain species 

 appear to prefer the neighbourhood of the towns, and may be 

 abundant therein without spreading into the surrounding country. 

 Thus, Sisymbrium Irio and Mercurialis annua have been long plenti- 

 ful about Dublin, but disappear a few miles beyond the city boun- 

 dary : Senecio squalidus is one of the most familiar sights of Cork ; 

 and Bromus madritensis is the botanical feature of Carrick-on-Suir. 



