262 



The Plant World 



BOOKS AND CURRENT LITERATURE 



Sernander, in a recent number o£ a Swedish botanical 

 publication, discusses the time of arrival in western Goetland of 

 Stipa pinnata, which was noticed by Linnaeus as a new element 

 in the Swedish flora in 1761, and had already been discovered 

 bv Falck in western Goetland. In the plant associations of 

 this district in which the species in question occurs xerophytes 

 and sen:i-xerophil mesophytes predominate. Their species as 

 regards distribution in Europe are southern and especially south- 

 eastern Thev are designated by the author as xerotherms, a 

 self-explanatory term corresponding to their habit and history. 

 Mcst of them as they occur in western Goetland are widely 

 separated from other stations in Scandinavia, and Stipa and some 

 others are also separated from their continuous area of continent- 

 al distribution. 



The Stipa associations appear to have entered western 

 Goetland by way of southern Sweden during a dry and warm per- 

 iod pieceding the present time Kerner, Engler and Drude hold 

 that there have been two dry periods in Central Europe in post- 

 glacial time; the first of these is designated by the author as a 

 sub-arctic steppe period and the second the xerothermic 

 period during which it is assumed that Stipa and certain of 

 its associates became established where they now are 



Two principal lines of evidence are adduced in support of 

 this, the first based upon the formation of peat bogs in Scandi- 

 navia^- the Aippeionost-layers of .which appear plainly to corres- 

 pond to" different iiimatic periods; : One of these was disting- 

 uished in- its'- dry 'climate, ; -which was also relatively warm, 

 during which pine" forests* existed in: central Scandinavia at least 

 100 meters higher on mountain slopes than at present, and the 

 hazel occurred farther north than now. At the same time the 

 area of peat bogs and swamps became less and forests encroached 

 upon them. 



The second line of evidence is derived from observations of 

 the effects of unusually dry and hot summers on the movements 

 of plants at the present time. In the dry summers of 1901 and 

 1904. for example, many plants on exposed rocks dried up and 



