572 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



for making taxonomic studies he should have available a good 

 regional ' flora ' and a well-arranged herbarium in which are filed 

 accurately-labelled specimens, dried and mounted on paper sheets, 

 of as many as possible of the entities involved. Only with such solid 

 foundations can plant geography really flourish and be of lasting 

 value to mankind. 



Different plant geographers work in diflferent ways, and even the 

 same investigator may use widely difl^erent methods in different 

 circumstances — according to the current state of knowledge, accord- 

 ing to when and where he was trained, according to what he has 

 set out to accomplish, and so on. If, as is often the case, the object 

 is to determine as far as possible the geographical range of a particular 

 plant species, the usual method is to have recourse to a major 

 herbarium. This should be done wherever possible because, 

 although perusal of appropriate literature may give a fair idea of 

 where a particular plant is present and where absent, it is nevertheless 

 desirable to check the range against actual specimens labelled 

 accurately and precisely with the localities in which they were 

 collected. Adequate travel for the purpose of determining the areas 

 of individual plants in the field is usually out of the question — and 

 unnecessary if a good herbarium is available in which the findings 

 of sufficient previous collectors are accumulated. Often, however, 

 it is necessary to visit (or borrow material from) more than one 

 herbarium for this purpose, bearing in mind the importance of 

 checking the identity of every specimen on which a fresh ' locality ' 

 is based, for the very best of herbaria will inevitably contain occasional 

 misidentifications and imprecisions. 



Concerning plant ranges it should be recalled that negative 

 evidence is insecure at best : because a particular plant species has 

 not been found in (or at least has not been recorded from) a given 

 region, we must not assume that it does not occur there — unless, 

 perhaps, a very limited and well-known area is involved.^ Nor 

 may we conclude, without detailed trials, that the plant in question 

 is incapable of growing in the area under the conditions obtaining 

 there. Thus, maps indicating the supposed ranges of individual 

 species can be misleading in suggesting absences which may not 

 exist. On the other hand, such maps are valuable especially when 

 showing (by spots) the definitely known stations. For these purposes 

 various outline maps are available ; but they must be ' spotted ' with 



^ Such instances as that illustrated in Fig. 62 should act as a warning in this 

 connection. 



