576 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



account the photoperiodic demands of some plants for a particular 

 length of day for such vital activities as flowering, and the special 

 needs of others for, say, water which in particular regions tends to 

 be available at particular seasons. 



This matching of geographical areas on the basis of purely physical 

 data should be verified by biological means, such as are provided 

 by phenological records of similar plants growing in the two or more 

 areas concerned. The plants used for such verification should if 

 possible be pure-line varieties or clones (that is, derived vegetatively 

 from a common ancestor) ; if the phenologies of given varieties of 

 Wheat, Potatoes, etc., are more or less similar in climatically and 

 latitudinally analogous areas, we then have our biological check that 

 these areas are agroclimatic analogues. Such preliminary tests may 

 conveniently be effected by exchange and co-operation among 

 agricultural experiment stations, as may the next desirable stage, 

 namely, establishment of uniform field trials of selected varieties. 

 Nor need there be limitation to two areas in this connection ; rather 

 is it desirable to have a world-wide indication of all areas of similar 

 phenology, meteorology, and latitude. This will give no guarantee 

 of successful introduction between the analogous areas, plants and 

 ecological factors being what they are ; but it could act as a useful 

 guide and save much time and eflFort, seed and money, which other- 

 wise can easily be wasted. A simpler device that may already save 

 some disappointment in introduction is the climograph. This is a 

 diagram constructed by plotting the mean monthly temperatures at 

 a point against their precipitation or humidity, and connecting the 

 twelve points for the year by a closed line. If approximately 

 coinciding climographs are given by two regions, at least for critical 

 periods of the year in so far as climate is concerned, it may be 

 considered worth while to try plant introductions from one to the 

 other. 



Many countries and states attempt to prevent the spread of plant 

 diseases into new areas by imposing rigid restrictions on the importa- 

 tion of living plant materials, for these often carry diseases. It is 

 particularly in fresh areas, or with species or strains that are unac- 

 customed to their ravages, that the greatest devastation by plant 

 diseases is likely to take place. A striking example is the virtual 

 extermination of Sweet Chestnut trees from eastern North America 

 by a blight introduced from China where it has long attacked with 

 far less eflFect a different but related species. Much the same 

 principles obtain, and quarantine or other restrictions are imposed. 



