Aug. 7, 1916 



Vegetative Succession Under Irrigation 



743 



short season; for although September often draws to a close before the 

 first killing frost, the next June may be half gone before the cottonwoods 

 along the streams have flaunted anything like full-grown leaves. Of 

 course, grasses and bulbous and thick-rooted perennials have beautified 

 the plains with flower or verdure long before the last frost is out of the 

 meadow lands, or at least before the spring freshets have subsided 

 enough to permit the growth of the meadow plants. 



PHYSIOGRAPHY OF ROCK CREEK REGION 



Physiographically the Rock Creek ranch is divisible into four regions 

 or units which for convenience may be designated and defined as fol- 

 lows: (1) The stream valley (PI. XCVIII), practically synonymous to the 

 flood plain of the creek and characterized by the natural meadows, 

 willow thickets, swamps, and cottonwood timber; (2) the bench slope 

 (PI. XCIX; C, fig. 1), representing the sides of the stream valley; (3) the 

 draws, or gullies (PI. C, fig. 2), occurring on the bench proper and break- 

 ing through the bench slope at intervals; and (4) the bench land (PI. 

 CI, fig. 1), flat, short-grass, upland plains. 



A knowledge is needed, of course, of the original flora of the flooded 

 lands, in order to comprehend the changes in vegetation which are 

 going to take place in some of the regions as the result of irrigation. 

 Accordingly the immediately succeeding paragraphs are devoted to a de- 

 scription and to an analysis of this flora. 



PHYTOGEOGRAPHY OF ROCK CREEK REGION 



The lists of plants under " Phy togeography of Rock Creek region" rep- 

 resent only those that contribute a present or later value toward the 

 working out of the problem in hand. For additional species noted, 

 mostly of interest only to the botanist, the reader is referred to the lists 

 of minor plants on pages 757-758. In some instances plants here 

 referred to under their generic name are given their specific designations 

 later and in these instances will not be found in the supplementary lists. 



BENCH FLORA 

 CHARACTERISTIC BENCH-LAND PLANTS 



Agropyron spp. (four species). 

 Duchloe dactyloides (Nutt.) Engelm. 

 Oryzopsis hymenoides (R. and S.) Ricker. 

 Eurotia lanata (Pursh) Moq. 

 Potentilla effusa Dougl. 

 Astragalus spp. (five species). 



Oxytropis monticola Gray. 

 Pentstemon angustifolius Pursh. 

 Chrysothamnus frigidus Greene . 

 Artemisia frigida Willd. 

 Lygodesmia jiincea Don. 

 Tetradymia inermis Nutt. 



The ecologist will note plants in this list representative of well-known 

 plant associations, such as the short-grass and the wheat-grass; the purpose 

 of these lists, however, is not to classify the plants of a given physiographic 

 unit but rather to treat such plants as a complex the limits of which are 



