22 A PLANT-DISEASE SL^EVEY IN TEXAS. 



a part of the low mountain country of the Edwards Plateau. A gen- 

 eral idea in regard to the character of the vegetation may be ob- 

 tained from the following quotation (4) : 



Its structure and habits indicate that it is a xerophytic or dry-climate vege- 

 tation : but though this is true of it as a whole, conditions vary enough to give 

 in some places, as in well-watered and sheltered canyons, a relatively luxuriant 

 growth, while in other situations, as upon stony, arid slopes, there is the 

 scantiest vegetation. 



One of the most characteristic features of this area is the extensive 

 cedar brakes along the Colorado from Austin to the northwest and 

 along the uj)per waters of the Llano, Guadalupe, Medina, Nueces, 

 and Frio Rivers. 



The greater part of the region lying south of a line connecting 

 Uvalde, San Antonio, and Skidmore is more level country, much 

 of which was formerly occupied by extensive, open grasslands, but 

 Avhich now shows over much of the territory dense thickets of 

 mesquite, cat's-claw, haujilla, huisache, whitebrush, platyopimtias, 

 cylindropuntias, and similar types of vegetation (5 and 8). 



It will thus be seen that the most productive portion of the terri- 

 tory lies largely to the east of a line connecting Austin, San Antonio, 

 and Skidmore, which includes the southern extension of the black- 

 land prairie and the timber belt of east Texas and extends through 

 the Fayette Prairie to the edge of the coast prairie on the southeast. 



RELATION OF DISEASES TO ENVIRONMENTAL FACTORS. 



The direct relationship between the environmental factors, both 

 edaphic and climatological, and the presence or absence of diseased 

 conditions of the vegetation is very evident. No one condition due, 

 perhaps, to the character of the soil is more noticeable than chlorosis. 

 It atfects practically the entire vegetation, being more evident in the 

 truck fields, nurseries, and peach orchards. No appreciable effect on 

 cotton and corn was noted. Since this chlorotic condition is limited 

 very largely to the Cretaceous prairies and the lime soils of the 

 Edwards Plateau, it is probably a lime chlorosis (38). It is known, 

 too, that continued droughts cause plants to become chlorotic. This 

 would explain in part the greater prevalence of this condition during 

 the past year. 



The downy and powdery mildews, which are quite common in 

 the States farther noi-th, are much less evident. The intense sunlight 

 and the high temperatures are no doubt responsible for this. The 

 spores of certain members of the Peronosporacea? are known to ger- 

 minate with difficulty in daylight and to exist only in shaded places 

 where the temperatures are excessive. The amount of sunlight can 

 be indicated and partially appreciated from the fact that the actual 

 number of clear or partly cloudy days ranged during 1909 from 



226 



