354 CARNEGIE INSTITUTION OF WASHINGTON. 



grasses or with sedges and rushes. Another remarkable type of park 

 is due to the xerophytic succession on long rock ridges. Probably the 

 most unique park of this sort occupies the Black Hills rim, the two 

 parallel ridges being covered with yellow pine and the intervening val- 

 ley with luxuriant mixed prairie. Similar though less extensive parks 

 are found in the Black Forest, which stretches eastward from Pike's 

 Peak over the watershed of the Arkansas and Platte Rivers. 



Periodicity in Vegetation, by F. E. Clements, E. S. Clements, and G. V. Loftfield. 



Several kinds of periodicity in vegetation have been studied during 

 the past three years. The simplest and most common of these is the 

 response of grassland to the wet and dry phases of the climatic cycle. 

 By a remarkable chance, the two or three years of exceptional drought 

 were followed generally by a year of the greatest rainfall known over 

 much of the West. These extremes were faithfully reflected in the 

 grasses and their associated herbs. One of the poorest years known 

 for grassland was followed immediately or a year later by the best 

 year, with the result that it was possible to make exact comparisons of 

 the two extreme conditions. This juxtaposition of extremes was so 

 graphic that it revealed many facts that might have remained obscure 

 in the more orderly progress of the climatic and vegetational cycles. 

 This was probably especially true in giving greater prominence to 

 relict groups and areas, and hence every advantage has been taken of 

 this opportunity to trace vegetational changes. A similar periodicity 

 has occurred in the case of shrubs and trees, but it is readily seen only 

 in the case of seedlings. 



Even a more striking type of periodic behavior has been shown by 

 the winter annuals of the desert plains of southern Arizona. The 

 very dry summer of 1918 was followed by normal winter rainfall, with 

 the result that the winter annuals developed in great luxuriance. 

 This was especially true of Eschscholtzia mexicana, the poppy. This 

 was so dominant over many square miles on and around the Santa 

 Rita Range Reserve that the brilliant mass could be seen 50 miles 

 away. Two other annuals, Lupinus sparsiflorus and Malacothrix 

 sonchoides, were frequent, but of minor importance. The next sum- 

 mer and winter were wet, and it was expected that the innumerable 

 seeds of the poppy would result in a greater crop than the year before. 

 It was a great surprise, in consequence, to find that the poppy was of 

 little or no importance over the plain, which was covered by alternes of 

 Gilia aurea, Franseria, and Orthocarpus, and other less abundant herbs. 

 The poppies retained their dominance only on the many bare mounds 

 formed by kangaroo rats. This furnished the plausible clue that the 

 grassy plain received too little heat at the time water-content was at 

 its best, and hence the poppy seeds germinated only on the wanner 

 mounds. Quadrats have been installed each year in the areas of 



