Note 31 — THE BISKRA REGION.

Briefly the case is as follows (see The root systems of desert plants, loc. cit.): Opuntia arbuscula growing near Tucson develops fleshy roots, but what is probably the same species growing about 100 miles distant has fibrous roots. Also, seedling opuntias have fleshy roots. Opuntia vivipara, which occurs naturally in the bottom of an arroyo (oued), may or may not have fleshy roots. By preliminary series of experiments it was learned that all opuntias tested which had an abundant water-supply developed fleshy roots, and it is assumed from this that the differences in this character as observed in nature had also such a physiological basis.