Butters and Roscndahl: effects of frost. 157 



frosts than any other vegetable tissues, it appears that our her- 

 baceous vegetation, and especially that of the prairie, is the sur- 

 vival from a long period of natural selection along these lines. In 

 herbaceous plant societies the advantage gained by a plant which 

 is able to survive such an experience over one which is killed back 

 to the ground and has to begin the season's growth anew, is 

 very great and this advantage has led to the almost complete ex- 

 termination of frost-tender herbs. 



(2). JVoody Vegetation. — Trees and shrubs, on the other hand, 

 suffered etxensive and more or less permanent injury. The follow- 

 ing figures are based on the examination of about 50 species includ- 

 ing a few common cultivated ones. The omission of the latter 

 would make no appreciable difference in the results. Less than 15 

 per cent escaped injury. In about 45 per cent the damage was lim- 

 ited to the leaves with occasional mechanical injury to the young 

 twags. In these the damage ranged all the way from a slight injury 

 to a few leaves up to almost complete defoliation. In about 30 per 

 cent the young twigs were much injured or wholly destroyed, 

 and in 10 per cent the old woody twigs also were killed or 

 injured — sometimes those of several years' growth (e.g. Quercus 

 niacrocarpa, Celtis).* 



The greater tenderness of our woody vegetation as compared 

 with our herbaceous plants is to be explained as due to two fac- 

 tors. (1). Between woody plants there is a less immediate com- 

 petition for light than among herbs. No tree can greatly over- 

 shade its neighbors by developing a few weeks ahead of them as 

 may easily happen in the case of herbaceous plants. There has 

 been, therefore, a much weaker tendency toward the elimination 

 of frost-tender trees than of frost-tender herbs. It is obvious, 

 however, that any frequent repetition of conditions as severe as 

 those of April, 1910, would permanently check the growth of some 

 of our more tender trees and would eventually lead to their 

 disappearance from our flora. (2). The general distribution of 

 our forest flora indicates that it has migrated from the south 

 and east and conditions of the sort which obtained in Minnesota 



*Tn Celtis all the new growth was destroyed by the first frost. After the 

 second frost the cambium and adjacent cells appeared black and watery in the 

 woodv twigs of 1909, 1908 and. for the most part, in those of 1907. Actual dea.th 

 of the twig-s was largely confined to the distal part of those of 1909. Enough 

 cambium cells remamed alive in the older twigs to start a new growth, but sec- 

 tions taken in December, 1910, show a ring of black undeveloped dead cells in 

 the midst of the wood for the year. 



