ROOT VARIATION IN DESERT PLANTS 



327 



VARIATIONS UNDER CULTURAL CONDITIONS 



Early in the spring of 1910 a plot of ground, about 16 m. 

 square, was set apart in the experimental garden of the Desert 

 Laboratory as a cactus garden in which it was proposed to ob- 

 serve the effect of cultural conditions on the root habit of the 

 plants introduced. The garden is on the western side of the 

 Santa Cruz River flood plain. The soil is adobe save along the 

 western border, where the experimental plot was located, which 

 has an admixture of small stones and sand. A mechanical analy- 

 sis of soil of the plot was made with the following result. 



An area in the center of the plot, 2 m. square, was excavated 

 to a depth of 1 m. and sand from a neighboring wash was used 

 to replace the soil removed. 



Throughout the 24 months which the culture was allowed to 

 stand the plot was irrigated once each week in the dry portions 

 of the spring and summer. The plants, therefore, had at all 

 seasons a good supply of water. 



The following species were planted, in north and south rows, 

 in the plot. First row. Opuntia arhuscula, from Sacaton, Ai'izona, 

 and 0. spinosior, from the bajada (mesa) near Tucson. Second 

 row: Opuntia discata, from Oracle, Arizona. Third row: Opuntia 

 toumeyi, from Tumamoc Hill, and 0. fulgida, from the bajada 

 near Tucson. Fourth row : Opuntia vivipara, from the type local- 

 ity near Tumamoc Hill, 0. discata, from Oracle, and 0. neoar- 

 huscula, from the bajada near Tucson. The second row passed 

 by the western edge of the sandy area in the center of the experi- 

 mental plot,' and the third row passed by the eastern edge of 

 the area. In all cases stem segments were set out and similar 

 treatment was given to all. 



Per Cent 



Coarser than 20 mesh' 



32 

 23 

 14 

 9 

 11 

 11 



20 mesh . 

 40 mesh. 

 60 mesh . 

 80 mesh . 

 100 mesh 



* Twenty to the inch. 



