X INTRODUCTION 



distinct idea, or set of circumstances; iinrorlunately, in practice, this is what 

 Watson's words do not express." 



A further feature in the occurrence of plants of transitory type lies in tiie 

 necessity of absence of competition which determines the floral cycle of a piece of 

 ground. The last writer indeed says : "A very little observation, properly directed, 

 will soon convince any unprejudiced mind, that freedom from competition is the 

 chief influencing cause of the appearance of annuals and biennials on freshly moved 

 soils. It settles the question of the duration of their stay; and everywhere, under 

 natural and artificial conditions alike, freedom from competition is the most potent 

 influence in the distribution of transitory species. 



"Man acts as a disturber of the floral cycles of nature, as a clearer and mover of 

 the ground, as an interrupter of competition, as a finder of elbow-room for annuals, 

 biennials, and the less powerful perennials. Directly or indirectly his influence is felt 

 in many other ways as a modifying force. Some have hardly been fully appreciated 

 yet. The one special manner in which every human being, by his actions or 

 through his wants, influences plant life, is as an aider and abettor of the weaklings 

 of our flora in their struggle for existence." 



Whilst most of the alien plants found in the British Isles colonize ground which 

 diff'ers from their natural habitat in their country of origin, some may establish 

 themselves here in closely similar habitats. In Mr. S. T. Dunn's Alien Flora of 

 Britain, the natural habitat of the species is given, but some of the plants included 

 in this work may, as the Rev. E. S. Marshall points out [Tlic Status of Some 

 Britannic Plants), be regarded as native in this country. The latter rightly 

 emphasizes the importance of correctly defining the width of meaning to be 

 attached to the somewhat loose term "waste places", and considers that in this 

 we should include village greens, rubbish heaps, unoccupied land near towns and 

 villages, and sandy commons, sea-shores, &c. The actual geographical distribution 

 in Europe of many species is complicated also by the existence of outliers. 



Of plants regarded as alien by Dunn, Mr. Marshall would exclude, among others: 

 Aconitum Napellus, possibly Pceonia corallina (though Mr. Druce thinks it is not 

 native), Lepidium Smithii, Sisymhrinm Sophia, Viola tricolor, Cerastium arvense, 

 Malva rotundifolia (near the sea). Geranium piisillum, Medicago 7ninima, Vicia 

 lutea, Pru7i!is ifisititia, Ribes Grossularia (in Yorkshire), Cotyledon Umbilicus, ^go- 

 podium Podagraria, Anthriscus vulgaris (on the coast), Sherardia arvcnsis, 

 Artemisia Absinthium, Chrysanthetnum Parthenium, Lactuca Scariola, Matricaria 

 inodora (vars.), Sonchus arvensis (seashores), Hyoscyamus niger (in woods), Veronica 

 arvensis, Ajuga Chamapitys, Atriplex patula (by slow streams), Chenopodium album 

 (in marshes), Rumex ptilcher (on the coast), Parietaria officinalis (on rocks and 

 cliffs), Galanthus nivalis (especially in the west), Apera interrupta. 



These are but a few of the plants whose nativeness has been questioned, that 

 may be regarded as indigenous. It is important to remember that a plant may be 

 native in one situation and casual in others. Thus it is necessary to make a careful 

 rock-soil analysis of records of each species, and to determine where and when it is 

 permanent, and when transitor}'. 



In considering the meaning of the terms species, varieties, &c., it is necessary 

 to admit the influence, not only of the inherent tendency to vary, but also of such 

 physiological factors as pollination (whether self-pollination or cross-pollination), 

 and of factors such as soil or climate. 



Variation in some genera, even in the British Flora, is extremely marked, and 

 whilst the normal number of species in a genus is four or five, there are some 

 genera that include as many as loo species (or sub-species or races). 



