[Chap. XXV TRANSPIRATION AFFECTS PLANT DEVELOPMENT 239 



plants, while shading and reducing the transpiration of the plants 

 beneath them, also have far more extensive root systems. Consequently 

 they may remove so much water from large areas of the soil that the 

 herbaceous and woody plants beneath with small or shallow root systems 

 die. 



Another example of the effectiveness of transpiration may be illus- 

 trated by the vegetation in gorges ( Fig. 81 ) . On the sides of the gorge 



Most Intense Li^Kt" 

 V Higher Summer Temperatures 

 |v^\. HidKeat Evaporation Rate 



Stream 



;5^ Most Intense Shade 



Lower Summer lemperaturcs 

 Lowest Evaporation Rate 



2 feet above t^c sod 



tli^hcst Ironspiratioa Rate 

 from low plants 



Fig. 81. Diagram of contrast in vegetation on north- and south-facing slopes of a 



gorge in Pennsylvania. 



are rock cliffs above long talus slopes. A narrow flood plain borders the 

 stream. To picture the extreme conditions we may consider a section 

 of a valley extending in an east and west direction. 



On top of the cliffs the upland hard pines are the only trees that have 

 survived. On the south-facing talus slope the mixed oak forest pre- 

 dominates, with scattered pines and hickories. On the steep north-facing 

 talus, hemlock, beech, birch, maple, and tulip are dominant. In the 

 upland surrounding this gorge the vegetation is oak, hickory, and pine. 

 Beech, hemlock, and birch have not survived above or on the south- 

 facing slope. Evaporation is several times as great there as on the 

 shaded, humid side of the gorge, and the water supply in the upper 

 layers of the soil is exhausted much sooner. Similar gorges where the 

 upland climate is too dry for trees such as beech, hemlock, tulip, and 

 birch may be found in many parts of the eastern United States. The 



