Botanical features of the Algerian Sahara

by William Austin Cannon

Observations of the leaf characters of the desert perennials, as shown in November, offer some points of interest, especially since there had been no rain for a year previous to the visit. As would be expected, the leaf-habit is various, ranging from scale-like or none to fair size. Several species appear to be evergreen, including Artemisia herba-alba, Capparis spinosa, Echium humilis, Fagonia bruguieri, Gymnocarpon sp., Haloxylon articulatum, Helianthemum sessiliflorum, Henophyton deserti, Herniaria fruticosa, and Salsola. Certain plants probably, as Henophyton, are facultative evergreens, retaining the leaves if the season is moist and dropping them if it is excessively dry; several of this genus were seen without leaves, but with green stems. (See, also, figs. [39,] [40,] and [41.])

Some idea of the variation in size of the leaves of a single species, as well as the average size of the leaves, will be had from the results of a few measurements. The leaves of Henophyton deserti range in length from 2 to 3.1 cm., and in breadth from 0.2 to 0.5 cm. The average length of 12 leaves from a single branch 7 inches in length was found to be 2.82 cm.; the average breadth was 0.35 cm. On another branch, 23 cm. long, were 35 mature leaves which averaged almost exactly the size of those just given. Since these leaves are rather numerous and of good size, it will appear at once that the species has a relatively large leaf-surface, even if the area can not be stated more definitely. (Fig. [30.])

The evergreen shrub Capparis spinosa has the largest leaves of any plant native to the Ghardaia region, and probably of southern Algeria. Its leaves are bilateral and nearly round and are placed upright on the branches; a series of 29 leaves from one branch 33 cm. long varied from 2 to 3.2 cm. in length and slightly less in width, the average being, length 2.9 cm., breadth 2.2 cm.

Euphorbia guyoniana has a deciduous leaf-habit. Its leaves are rather small but numerous. A shoot 22 cm. long bore 40 leaves varying in length from 1.5 to 2.1 cm. and in breadth from 1.2 to 2.0 mm., with the average length and average breadth 1.7 cm. and 1.6 mm. respectively.

It is worth noting that the surface of the leaves, as shown by inspection, is, possibly, most often smooth, as, for example, in Capparis, Henophyton, and Haloxylon. Dense hairiness is an exception among the species seen, Lithospermum callosum being the only plant striking in this regard.

GROWTH AND FLOWERING HABITS IN THE GHARDAIA REGION.

Upon arriving at Ghardaia in November, it was surprising to find several species of perennials, under strictly desert conditions, putting on new growth, forming fresh leaves, or coming into flower. Especially was this unexpected in view of the fact that no rain had fallen in the region for twelve months. A study of the roots in relation to the depth to water showed also that most of the native plants, during the dry season, could not have penetrated to a depth anywhere near that of the water-table of the valley, and the water relations of the plants growing on the plain above the M’Zab Valley were even more severe. Further, most of the species do not possess water-storage faculty. It should be noted that the only apparent difference in the water relation between November and in early autumn, or summer, was that of raising the relative humidity of the air through the lower temperature. In a preceding section it has been stated that little or no evaporation took place at night in November at Ghardaia, and possibly less in the daytime than would have been supposed. The leading environmental changes were, of course, the lower daily temperature and the really cool nights.

It is not uncommon in the Arizona desert for a species to form flowers or leaves, following a change in temperature, the moisture conditions being not otherwise changed, but, so far as I know, the temperature changes bringing about this result are always from a cooler to a warmer condition and not the reverse. It would not be expected, consequently, that in the present instance a renewal of vegetative activities would follow as a direct result of a lower temperature, although analogous changes are necessary before certain species, after rest, will start development. Whatever may be the immediate cause of the renewal of activity on the part of the plants at Ghardaia as noted, the following species were seen to have formed new leaves: Henophyton deserti and Zilla macroptera; also the following fresh flowers with or without shoot growth: Fagonia bruguieri, Haloxylon articulatum, Henophyton deserti, Ononis polyclada, and Zollikoferia resedifolia, and three other species not determined.

GHARDAIA TO TOUGGOURT.

From Ghardaia the route, consisting of camel trails only, pursued a course south of east to Ouargla, and from Ouargla a direction east of north to Touggourt, over 400 kilometers, Ouargla being about half-way. There are no villages between Ghardaia and Ouargla and none between this place and a point 20 kilometers south of Touggourt, so that in accounts of the vegetation or the topography, lacking convenient points around which to center descriptions, the device will be adopted of using distance estimations to or from the three chief towns.