574 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



The introduction of exotics — for crops, mere convenience, or 

 aesthetic or other purposes — is an appHcation of importance. Species 

 introduced into a new region or at all events a new environment 

 may vary in their fate from complete failure of growth to becoming 

 pests and largely ' taking over ' their new locale, after the manner of 

 the Prickly-pear in Australia, though this is usually only where 

 (and not much longer than) there is drastic disturbance by Man. 

 Nevertheless introductions should be effected with care, preferably 

 after appropriate trials have been made. The result may of course 

 greatly extend the area inhabited by the species concerned. Thus 

 many of the finest coniferous forests of Europe are now of Douglas 

 Fir or other western North American species, and some considerable 

 plantations in the eastern United States are of Scots Pine. In 

 making such introductions it is prudent to consider not only the 

 conditions in the lands concerned but also the ecological economy 

 of the individual plants. For example, certain American trees thrive 

 only in mixed stands, and so application in America of the pure- 

 stand tendency of European forestry is liable to be unsuccessful 

 where these species are concerned. 



In connection with plant introduction, consideration of what are 

 known as ' agroclimatic analogues ' may be valuable. These are 

 based on the principle that a given variety tends to be very similar 

 in its phenological behaviour (that is, with regard to such weather- 

 affected activities as the times of flowering and fruiting) in areas of 

 similar climatic and latitudinal conditions, the analogues being areas 

 that are sufficiently alike in these features affecting crop production 

 to offer a fair chance for success of plant materials transported among 

 themselves. Elements of comparison in determining these analogues 

 are the mean monthly and yearly temperatures, absolute minimum 

 and maximum temperatures, average monthly, seasonal, and yearly 

 precipitation, precipitation-evaporation ratios, length of frostless 

 periods, and latitudes. In addition, soil and other features have of 

 course to be taken into consideration when making trials. Fig. 

 184 indicates the application of such comparison to the Ukraine 

 in terms of United States districts, with distinction between year- 

 round analogues (for winter and spring crops, the appropriate State 

 being named in slanting type) and April-October analogues (for 

 spring crops only, the State being named in vertical type). In other 

 such maps available at the American Institute of Crop Ecology, 

 Washington, D.C., the winter and spring crop analogues may be 

 separated. Consideration of latitude and the time of year takes into 



