Plant Migration Studies 5 



borders of some streams, the increase of organic matter in the 

 drainless valleys of the Sandhills, and the alkali soils surround- 

 ing many ponds in regions still further west. 



The climate of Nebraska is of the "continental" type. The 

 rainfall which reaches 88 centimetres (35 inches) a year in the 

 southeastern part gradually decreases westward to 35 centimetres 

 (14 inches). It is very unequally distributed throughout the 

 }-ear. About 30 per cent falls in the spring, 39 per cent in the 

 summer, 23 per cent in the autumn, and 8 per cent in the winter. 

 The humidity of the air is generally low, and is especially so in 

 the winter. The msolation is high, the days with sunshine be- 

 ing more than three times as many as those without. The tem- 

 perature ranges are from about 38°C. (ioo°F.) as the maximum 

 heat of summer, to — 36 °C. ( — .30 °F.) as the minimum of win- 

 ter, the former for the southern counties, and the latter for the 

 northern. The prevailing winds are from the southeast in spring 

 and summer, and from the northwest in the autumn and winter. 

 The average for ten years of the number of miles of wind for 

 each season in eastern Nebraska (Lincoln) is, 28,111 in spring, 

 21,016 in summer, 23,586 in autumn, and 23,460 in winter. 



The native trees of Nebraska have developed many devices, 

 for the distribution of their seeds, adapted to the physical factors 

 just described. These may be reduced to five general classes, 

 viz. : wings, hairs, fleshy fruits, rolling balls, edible nuts. 



WINGS 



Rock pine (Pinus scopulornni (Engelm.) Lemmon). Each 

 seed is provided with a deHcate membranous wing, a centimetre 

 long and five to seven millimetres wide. When the seed drops 

 from the cone it is given a whirling motion by a slight twist and 

 bend in the plane of the wing, and if caught by the wind is car- 

 ried a considerable distance from the parent tree. This tree oc- 

 curs in the Rocky Mountains from Montana to Wyoming and 

 Colorado, and in Nebraska (i^) has pushed out upon the foothills 



' The figures in parentheses refer to the maps showing the distribution 

 of the different species of trees. 



15 



