[25]The Root Habits of Desert Plants, l. c.
[26]Compare the root-system of the species at Biskra, p. [64].
[27]In the vicinity of Tucson (see The Root Habits of Desert Plants) is to be found a slender-stemmed Opuntia whose roots are fleshy and are placed within 2 to 4 cm. of the surface of the ground. It has been observed that if the roots are examined in the midst of a dry season, as in June, they are gorged with water, but if the soil is removed for a few hours they become shriveled. A similar habit was seen in another species of the same genus. Two other genera of the cacti from the Tucson region have the water-storage organs wholly or partly protected by the soil. In Cereus greggii the subterranean portion forms an organ 15 to 30 cm. in diameter, and in the other form the fleshy subaerial stem is partly drawn under the surface of the soil, so that only the flat upper surface is visible.
[28]In the case of annuals the differences in development of the shoot between plants well watered and those with only a meager supply are very striking. In one instance in the Tucson region specimens of Parietaria debilis growing in extreme conditions, one moist and the other arid, varied in length between 39 cm. and 8 mm., or a difference with a ratio of 49 to 1. (Root Habits of Desert Plants, loc. cit.)
[29]The vegetation in the vicinity of Biskra is so well known that a sketch will suffice as a basis of comparison with the flora and conditions of plant life farther south.
[30]Liste des plantes observées aux environs de Biskra et dans l’Aurés, Trabut et al., Alger, 1892.
[31]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.
[32]Eine botanische Exkursion nach Algier and Tunis, Bericht der Senckenbergischen Naturforschenden Gesellschaft in Frankfort a. M., p. 76, 1910.
[33]Plants with subterranean water-storage organs—bulbous plants—are said to be a feature of the High Plateau.
[34]Through Timbuctu and across the Great Sahara, 1912, p. 266.