E. V. Wulff —148— Historical Plant Geography 



fied to by the finding, in Pliocene deposits in central France, of leaves 

 identical in appearance to those of our contemporary blueberries 

 {Vaccinimn uliginosum) , which to-day still grow on these same moun- 

 tains in France at an altitude of 1,500-1,600 m. (Laurent, 1904-1905). 



Daphne Julia, a Tertiary relic which survived the Ice Age and still 

 grows in the steppes of Voronezh Region, after a rainy summer retains 

 its leaves until the next vegetative period. Hence, the evergreen nature 

 of the genus Daphne has not been entirely lost by D. Julia, though it is 

 entirely alien to the steppe vegetation and to the climatic conditions 

 now existing in these steppes (Koso-Poljansky, 1928). 



An analogous case of physiological atavism we find in the European 

 species of oak and beech {Qu^rcus Rohur, Q. sessiliflora, and Fagus sylva- 

 tica). It has long since been established that some specimens of these 

 trees, particularly of these species of oak, retain their foliage (in a 

 dead state) until the spring of the following year. This gave grounds 

 for distinguishing between "summer" and "winter" forms. How can 

 we explain this retention of leaves, when all other deciduous species 

 lose their foliage as soon as the weather turns cool in autumn? The 

 only explanation we can give of this phenomenon is that these species 

 in past times dwelt in a different, warmer climate, not characterized by 

 such marked periodicity as now prevails in the northern latitudes, and 

 they were, therefore, enabled to live as evergreens, i.e., not losing their 

 foHage all at once at a definite period but only gradually, a few leaves 

 at a time, simultaneously developing young leaves to replace the old. 

 The correctness of this assumption is confirmed by anatomical data. 

 Usually in deciduous species there may be observed toward autumn, 

 below the points where the leaf-stalks are attached, the development 

 of cork (suberose) tissue and of a special, loose layer of cells, as a re- 

 sult of which the leaf and jjetiole, separated from the branch (to which 

 it formerly was firmly attached) solely by this cork tissue, easily drop 

 off. The place of leaf abscission is already in advance covered by this 

 layer of suberose cells. Thus, while ordinarily defoliation is a purely 

 mechanical process, due to the suberization and dying away of tissues 

 at the point of leaf abscission, in the above-mentioned species of beech 

 and oak the place of abscission remains alive and green during the 

 entire winter. 



Further evidence of the above assumption is the fact that the time 

 of the falling of the leaves in these species of beech and oak coincides 

 with the development of new leaf-buds, as is the case in evergreen 

 plants. In the latter the leaf falls at the precise moment that the bud 

 in the leaf's axil begins development. These species of oak are closely 

 related to evergreen species, and we have full grounds for the assump- 

 tion that the above-mentioned biological peculiarities show that these 

 species in the past were adapted to different climatic conditions. This 

 is further confirmed by the fact that in southern Europe Quercus robur 

 and Q. sessiliflora sometimes retain their leaves in a green state through- 

 out the winter. Hence, even after a period of thousands of years, these 

 species of oak and beech, as regards biological periodicity, have not yet 

 become adapted to the climatic rhythm in their present habitats 

 (Magnus, 1913). 



In this connection it is of interest that the American plant anato- 



