PAMMEL NOTES ON GRASSES OF NEBRASKA, ETC. 235 



"In northward distribution terrestrial animals and plants are 

 restricted by the sum of the positive temperatures for the entire season 

 of growth and reproduction. 



"In southward distribution they are restricted by the mean temper- 

 ature of a brief period during the hottest part of the year." 



These principles cover the fundamental facts. 



Some species have adapted themselves to a wider range than others. 

 This is shown very nicely in such species as Festiica ovina, which 

 occurs at an altitude varying from 4,500 to 10,000 feet. We note, 

 however, that soil has an important be.iring on its distribution. 

 Agropyron spicatuin is perfectly at home on the plains of Nebraska 

 and adjacent foot-hills of the Big Horn Mountains, but at an altitude 

 of 7,500 feet it is not so common. Here it has nearly reached its alti- 

 tudinal limit of distribution. Its occurrence in and about Rapid 

 Creek Park on dry slopes is not peculiar since the sunny sides of the 

 parks receive an intense heat during the day. The soil is dry and 

 this accounts for its full development. The Poa Bucklcyana seem- 

 ingly has a considerable range of altitude. Its occurrence at New 

 Castle at an altitude of 4,019 feet is not strange, for many of the j^lants 

 of this interesting region are northern. The nights are cool and the 

 timber has greatly modified the excessive heat of the adjacent plains. 

 At Dome Lake, 9,200 feet, this species occurs on the moraines. The 

 soil, as I have said elsewhere, is of a gravelly nature, the surface being 

 very dry. The Agropyron tenerum is not only common at Hastings, 

 Nebraska, altitude 1,943, Grand Island, 1,872, and Broken Bow, 2,488, 

 but along the irrigation ditches and streams at an altitude somewhat 

 less than 5,000 feet. Tht A. psei/do-repens has a wider distribution. 

 It is not uncommon in the prairie states, Iowa, Colorado and Nebraska. 

 It occurs not only at Broken Bow at 2,480 feet, but in the grassy parks 

 at 5,500, and 8,500 feet of the Big Horn Range. But moisture is the 

 factor which determines its distribution at a lower altitude. A more 

 interesting case of the influence of moisture on the distribution of spe- 

 cies is to be found in souie of the s|jecies of Foa collected on the trip, 

 namely, Foa arctica and F. Whcclcri. The altitude of F. arctica is 

 7,500 feet, not, however, in connection with Savastana odorata, which 

 is abundant in the open, wet, grassy meadows, but with Foa Wkceleri 

 and F. lepfocoma in the cold spagnaceous swamps and canons where 

 the Engelmann Spruce occurs. In Northern Colorado the writer* 



* L. H. Pammel, Ic. Bull. Div. of Agros. 7:41. 



