E. V. Wulff —108— Historical Plant Geography 



in the geographical distribution of plants. In this connection Willis 

 (1922) quite correctly points out that acclimatization does not neces- 

 sarily involve the transfer of plants from one place to another. Dur- 

 ing the history of vegetation plants have many times had to become 

 acclimatized in situ to the great climatic changes that have occurred 

 during geological ages. Willis sums up his chapter on this subject as 

 follows (p. 31): — 



"Acclimatisation in the hands of man, who is impatient of results, has been largely a 

 matter of trial and error, with numerous failures, but there is reason to suppose that this 

 is not so much the case in the hands of nature, working as she does over vast periods of 

 time, with very small steps. Species have thus been acclimatised to conditions wonder- 

 fully different from those in which they began". 



In other words, in studying the artificial factors in plant distri- 

 bution we should keep in mind that these embrace not acclimatization 

 but only naturalization, a first step in acclimatization, this constitut- 

 ing a limitation, and a very real one, to the possibilities of introducing 

 new members into a flora at the will of man. 



As regards naturalization, numerous cases are known where this has 

 taken place or is taking place, which indicates, as de Candolle 

 pointed out, that in the various regions of the earth there do not grow 

 by far all the species which these regions could maintain and which 

 could thrive there. At the same time, however, it is impossible to draw 

 a sharp distinction between acclimatization and naturalization, since, 

 though there are regions with similar climatic conditions, there are no 

 two regions in the world with entirely identical conditions, and, conse- 

 quently, a plant must pass through a number of long stages in the 

 process of becoming adapted to new conditions before it may be re- 

 garded as fully naturalized. These stages of gradual adaptation to 

 new conditions are particularly clear as regards climatic, especially 

 temperature, conditions. As an example, we may take the olive trees 

 {Olea europaea) grown in the Crimea. They are the descendants of a 

 number of very old trees preserved from ancient Greek and Genoese 

 settlements. These trees undoubtedly represent cold-resistant races 

 that evolved on the southern coast of the Crimea, the northernmost 

 limit of olive cultivation. 



The various stages in a plant's adaptation to low temperature may 

 be illustrated by the acclimatization studies carried out at the Nikita 

 Botanical Gardens in the Crimea (Wulff, 1926). We divided the alien 

 plants there into four different groups according to their stage of 

 naturalization. The first group included plants fully naturalized (in 

 DE Candolle's sense), i.e., capable of self -propagation, growing outside 

 the Hmits of cultivated fields, and, in general, having acquired the 

 attributes of indigenes. (As examples we may mention: Daphne 

 laureola, Madura aurantiaca, Mahonia aquifolium, Cylisus laburnum, 

 Bupleurum frulicosum). In the second group we assembled those 

 species that propagated themselves freely by seeds in the plots where 

 they were cultivated but that did not spread beyond these plots and 

 did not revert to a true wild state. (Examples: cedars, Trachycarpus 

 excelsus). The third group, including the majority of the plants in the 

 Nikita Botanical Gardens, embraced those species that fruited normally 

 and produced viable seeds but that were incapable of self -propagation. 



