552 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



then, in order for the species to become estabhshed in any numbers, 

 it would have to reproduce successfully and withstand predators, 

 diseases, and competitors. Alternatively, Man may have taken a 

 hand, though, as we have often seen, in the case of introductions 

 his continued intervention is commonly necessary for persistence. 

 In any case, a whole complex of circumstances lie behind the arrival 

 of a particular plant in a particular place, and a further complex 

 of conditions has to be satisfied before it can become established and 

 survive there. Furthermore, for anything approaching permanence, 

 we have the many problems involved in successful reproduction. 

 And as both circumstances and conditions are complicated, and it 

 seems safe to conclude that there are very many thousands of different 

 combinations of them favouring the existence of particular species 

 among the hundreds of thousands known on earth, it would seem 

 futile to attempt to treat these matters systematically here. Even 

 a series of specific examples from a particular place would be of 

 little help, unless it were to inhabitants of that one place — especially 

 as in the final analysis a good deal is left to chance. 



What the asker of the above question may often have in mind, 

 however, is ivhat does the presence of a particular plant tell lis about 

 the place in which it grows — especially in terms of practical applica- 

 tion ? It usually means that the plant involved has found suitable 

 conditions at least for growth and survival, and that the conditions 

 under which we may know it to flourish elsewhere are approximated 

 to here — though we should meanwhile recall that preferences may 

 change {e.g. at different stages of the life-cycle), as may of course 

 ecological factors themselves. Although many plants have too wide 

 an ecological amplitude to mean much in this connection, at least 

 as far as present-day knowledge goes, others are exacting in their 

 requirements. Others, again, have specific needs such as the 

 presence (or absence) of particular substances in the soil. Here 

 we think immediately of certain ' indicator ' plants or groups of 

 plants which are so called because they require (and therefore 

 indicate) the presence of particular conditions, and may consequently 

 be valuable in demonstrating that these conditions obtain where 

 they grow. For the existence of a particular plant or community 

 in a particular place is an expression within certain limits of a 

 particular combination of actual habitat conditions, among other 

 things, and, where these limits are narrow, or a specific one is finite, 

 we have a ready-made demonstration of the situation obtaining 

 locally. 



