224 JOURNAL OF THE ARNOLD ARBORETUM (vol ii 



accordinfz to their ranks and requirements, from the hydrophytes in tlie 

 pools to the SelaginelUis and Lichens upon the almost naked rocks. Fol- 

 lowing; a rainy season the annuals spring up quickly under the stimulus 

 of the liot sun, pass rapidly through their life cycles and deposit their 

 seeds to lie dormant until favorable conditions again ensue. To the 

 taxonomist and ecologist the barrens furnish rich fields for exploration 

 and study, on account of the many rare plants found in them and the 

 peculiar conditions under which they grow in these bits of semi-desert 

 isolated in the midst of a well watered forest land. But even more in- 

 teresting are the curious problems in plant geography and in the phyto- 

 graphic history of the region which they suggest, and upon which they 

 may throw some light. A study of the peculiar plants of the barrens 

 shows many of them to be identical with or closely allied to species char- 

 acteristic of the western plains or of the semi-arid southwest; and their 

 occurrence in the Ozark region, separated in some cases by hundreds of 

 miles of distance and apparently insurmountable ecologic barriers from 

 the main areas of their present range challenges the inquiring mind to 

 theorize upon the questions of how they come here; whether they re- 

 present the advance guard of an eastward extension of the plains floras 

 or are survivors of a former occupation; and if the former by what extra- 

 ordinary means were tliey transported here, or if the latter what great 

 climatic or cculogic changes must have occurred to drive the plains plants 

 back so many degrees westward and permit the development of so differ- 

 ent a type of flora here.^ 



Closely allied to the floras of the barrens and their interesting pro])lems 

 are thos-' of the bare-topped hills found in certain sections of the western 

 Ozark ) plateau. Over considerable areas, most typically developed in 

 Stone, Taney and Ozark Counties, Missouri, much of the surface has 

 been carved into series of low, dome-like, rounded or flat-to])pcd hills, 

 divided by a net-work of ravines and narrow valleys, some of them trav- 

 ersed by spring-fed streams, but mostly dry excej)! for short periods after 

 rains when they are flooded by the run-ofT from the hills. This peculiar 

 topography is due to* the character of the underlying dci)OsiLs, made up 

 largely of thin horizontal l)eds of moderately resistent limestone alternat- 

 ing with softer hiyers, mainly clay and decomposed cidrareous material 

 with some chert. Ledges of the limestone frequently stand out, forming 

 terraces entirely surrounding the hi'ls. There is usually a rather dense 

 growth of trees, shrubs and vines in the intervening valleys, and some- 

 times this extends for some distance up the slo[)es of the hills especially 

 on the northern and eastern sides, but often the upper portions and tops 

 are nearly or entirely destitute of woody species, and support a charac- 

 teristic herbaceous flora. On some of the knobs, however, a few shrubs 

 or stunted trees may be st^^n following ravines or ledges well up toward 

 the sununits, or snudl colonies may have established themselves upon the 

 level tops. Amongst the conunouest woody species in such locations are 

 Jnnipenis virginiaua L., Qurrcus viarilandica Muenchh., Qutnus Shu- 



