98 



PLANT LIFE OF ALABAMA. 



The last killing frost happens between the middle of February and 

 latter part of April, on the average, about the 8th or 10th of March. 

 The maximum temperature was in July 106 ; the minimum in Janu- 

 uary 5 . 1 The course of the temperature, which has such a potent 

 influence on plant growth, is best represented in the following series 

 of the absolute and average minima and maxima for each month dur- 

 ing the same period: 



Series of maximum and minimum temperatures. 



XEROPHILE FORESTS. 



Post-oak prairies. All over the region, particularly on the highest 

 swells, the drifted clayey loams, sands, and pebbles mingle with the 

 soil resulting from the decomposition of the underlying limestone 

 strata, and give rise to light, more or less sandy, loams or heavy cal- 

 careous clay soils, covered with open xerophile forests of Spanish oak, 

 black oak, black jack, post oak, the last predominating, associated 

 with mocker nut and pignut hickory. Where the heavy clay soils 

 prevail, destitute of humus, water-soaked during seasons of rain and 

 baking to a hard crust during droughts, these so-called post oak prai- 

 ries have been but little encroached upon. Under the cover of the 

 brush the gramineous plant formations predominate, while sedge 

 grasses (Cyperaceae) abound in the more shaded and wet situations. 

 For example, Eleocharis tuberculosa, E. ovataj- and Oyperus pseudo- 

 vegetits* on the borders of ditches and pools; Ca/rex triceps, a very 

 distinct southern type, occurring from South Carolina to central 

 Texas, but not in this State north of the southern edge of the meta- 

 morphic hills; C. hirsuta* C. oblita, C. sterilis? C. lurida? common 

 throughout the Carolinian area; C. oxylepis, C. cherokeensis, C. debilis 

 pubera of a more southern range; together with Panicum melicarmm, 

 P. anceps, and Uniola laxa. 1 In drier and more exposed places: 



Scleria triglomerata 1 (abundant). Steironema dliatum. 1 



Allium canadense. 1 Steironema lanceolatum. 1 



Quamasia esculenta. 1 Penstemon hirsutus. 1 



Lobelia appendiculata. Hibiscus incanus. 



The last is a stately perennial of the Louisianian area known from 

 a few localities between South Carolina and northwestern Louisiana. 



1 See footnote *, p. 26. 



2 Rarely found north of the Central Pine belt. 



