14 AGRICULTURAL AND MECHANICAL COLLEGE OF TEXAS. 



luxuriant for agriculture. This is only in the main true. Many species 

 of trees when once well established grow luxuriantly over portions of 

 the .Great Plains and the Central Basin, where the rainfall does not exceed 

 20 or 25 inches a year. When suitably cultivated, trees are grown with- 

 out irrigation in the extreme northwestern Panhandle, where the rain- 

 fall is less than 15 inches. Eainfall alone does not determine the limits 

 within which forest types are found but is undoubtedly the most im- 

 portant controlling factor, as is shown by the gradual transition from 

 moisture-loving to drouth-enduring species which takes place along the 

 line of decreasing rainfall. 



The nature of the soil and rock have an important influence in the 

 distribution of tree growth. Heavy timber in the canyons of the 

 Edwards Plateau, on the broken strata of mountain masses and along 

 breaks in regions of low rainfall is accounted for by the supply of per- 

 colating waters made available through the character of the rock struc- 

 ture. On the other hand, the waxy clay soils of the Black Prairie, 

 which tenaciously retain moisture supplied by an abundant rainfall, are 

 not generally forested. Natural treeless prairies are characteristic of 

 the southeast Texas Gulf section and large portions of the Fayette 

 Prairie and Lignitic belt, which cannot be accounted for by lack of 

 rainfall. They are explained in part at least by the impervious, com- 

 pact condition of the soil which does not furnish sufficient air to satis- 

 factorily supply the roots of many kinds of trees unless it has been 

 "broken up and made mellow and porous by cultivation. Competition 

 with native grasses also helps to exclude tree growth on these areas. 

 Lack of aeration of compact soils has a larger influence in determining 

 tree distribution and types of forest in many parts of the Coastal Plain 

 than any other factor, rainfall not excepted. 



Temperature is a dominant selective agency so far as the adaptation 

 of species to regions varying widely in length of growing season, dates 

 of early and late frosts and in duration and severity of periods of cold 

 weather are concerned. It distinguishes the flora of the Coast country 

 from that of North Texas and characterizes the flora of the State as 

 a whole. 



Texas presents extremes in the amount and intensity of sunshine 

 which are reflected in the character of the forest growth. The moist 

 atmosphere and the high percentage of partly or wholly cloudy days in 

 the eastern region prevent the intense illumination found in the regions 

 farther west and the forests develop as a consequence thick crowns and 

 a heavy undergrowth of shade-loving species. Except in moist canyons 

 the western forests are conspicuously wanting in shade-loving and dense 

 foliaged species. 



In the Coast Plain country and on the Great Plains the wind tends 

 to hinder the spread of timbered areas and on exposed situations results 

 in trees of low form. The periodical occurrence of Gulf hurricanes has 

 a profound effect on the history of forest extension and the present aspect 

 of the Coast Plain forests. The ragged, uneven growth of timber in 

 the Coast countrv is the most evident effect of these storms. On the 



