38 



The Plant World. 



Higher strata of peat contain remains of the oak, while the 

 deposits nearer the surface reveal remains of the European 

 beech. Thus three successive forest growths are represented 

 in Denmark, beginning with the time when the peat began its 

 formation. The character of the forests gives us a means of 

 estimating the secular changes of climate. Norwegian botanists 

 have led in research along these lines. Jens Holmboe,* one of 

 the most recent laborers in this field, has reviewed the work of 

 Axel Blytt, Nathorst, Rekstad, Gunnar Anderson, Sernander, 

 A. Schulz and others, and substantiates their views as to the 

 periodic changes of climate since the end of the glacial period, 

 as revealed by an examination of the plant remains in peat 

 bogs. He recognizes ninety species of plants which have been dis- 

 covered in the bogs of Norway. After a critical study of the 

 remains as found in situ in the peat deposits, Holmboe recog- 

 nizes the following successive strata: (a) dwarf birch, polar 

 willow (Salix herbacea) and water plants; (b) birches (undoubt- 

 edly Betula odorata) which formed the primeval postglacial 

 forest, similar to the forest which is represented today at the 

 northern limit of trees together with aspen, tall willows, juniper, 

 bilberry, white water-lily {Nymphaea alba) ; (c) pine stratum 

 with alder, hazel, raspberry, twin-flower (Linnaea borealis) and 

 twig-rush (Cladium mariscus), while in the lowest parts of this 

 stratum with the pine are found remains of dwarf willows and 

 avens (Dry as octopetala) ; (d) the layer of the oak with the ash, 

 Norway maple, hazel and winter linden (Tilia parvifolia), these 

 trees being found in a broad belt during the warmest post- 

 glacial period, forming in many places extensive forests; (e) the 

 layer of Pinus silvestris (Fichtenzone) which occurs only in a 

 few places in Kristiania and Trondjemsf jord and represents the 

 latest fossil layer of peat; (f) then comes last, the heath stratum 

 characterized by the heather (Calluna vulgaris), which exists 

 in great abundance on the treeless west coast of Norway, replac- 

 ing the earlier forests as the most important formation plant 

 Similarly K. R. KupfTer found just above the clay bottom of a 

 bog in Russia, a layer of sand ten to fifteen cm. thick, filled 

 with the remains of characteristic alpine plants, such as Dryas 

 octopetala and willows (Salix arbuscula, hastata, herbacea, polaris, 



* Drude, O. Bericht ueber die Forschritte in der Geographic der Pflanzen (1901-04) 

 Geographiscb.es Jahrbuch XXVIII : 214-219. 1905. 



