90 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Table 19. — Average daily evaporation on the gravel-slide and half-gravel-slide during 1918. 



One factor greatly accelerating evaporation is the marked wind 

 movement. From June 6 to August 23 the average daily wind velocity 

 at a height of 0.5 m. was 103 miles. Notwithstanding the high evapora- 

 tion losses, nearly all of which occur during the day, the upper layer 

 of the soil containing the roots is kept moist by frequent rains. Seven- 



^•i-:* 





Fig. 37. — Quadrat-bisect showing root relations of gravel-slide plants. This was made 

 along the front of the quadrat shown in plate 23, b. P , Paronychia jamesii; A, Aletes 

 acaulis; K, Krynitzkia virgata. 



teen showers, varying in intensity from 0.1 to 1.85 inches, occurred 

 from June 21 to August 28, 1918, while the total rainfall during the 

 period was over 10 inches. As already pointed out, none of this water 

 runs off except during the heaviest rains, while the loose gravel inhibits 

 high evaporation losses. Moreover, the plants are so sparsely spaced 

 that only relatively small amounts of water are removed by transpira- 



