GENERAL SUMMARY AND CONCLUSIONS. 77 



ROOT-CHARACTERS AND SPECIES DISTRIBUTION IN SOUTHERN ALGERIA. 



The general principles bearing upon the relation between the type of 

 root-system and the distribution of the species, as observed in southern 

 Algeria, can be briefly stated. Often the relation is close and apparent, 

 but not always. For example, large perennials, such as Tamarix and 

 Zizyphus, have an obligate specialized root-system, with a long tap-root. 

 These plants naturally occur where there is considerable depth of soil, and 

 hence are not to be found on the hamada, for instance, where it is shallow. 

 On the other hand, such species as have a generalized root-system, like 

 Acanthyllis and Haloxylon, are to be found on the hamada, but they occur 

 also in other habitats where the soil is deep. The last type of root-system 

 is flexible, accommodating the species to a wide range of soil conditions. 

 In doing this the change in form is almost a change in type; for example, 

 the roots of Haloxylon on the hamada at Ghardaia develop both laterals 

 and a main root, but in deeper soil, as at Biskra and Ghardaia also, the 

 laterals are nearly suppressed and the tap-root is the striking feature. A 

 marked exception to the rule that plants with a generalized type of root- 

 system have also the widest local distribution lies in Peganum harmala, 

 which, having roots of this character, is nevertheless restricted to habitats 

 where the soil is deep. The species is a half-shrub, having a perennial 

 subterranean portion and a short-lived subaerial portion, the life of which 

 appears to depend on the character of the water-supply. As learned by 

 Fitting, the species can develop in its leaves a very dense sap, enabling 

 it to extract water from a very dry soil.* In spite of this fact it appears 

 to act like an annual in certain regards, requiring at all times, particularly 

 during the most arid season, a relatively good water-supply. 



Thus, in brief, a study of the relation of the root-type of the Algerian 

 plants to the plant's distribution leads to the same general conclusion 

 already obtained by similar but more extended study in the Arizona desert, 

 namely, that the connection is often a very close one and often of definitive 

 importance. Where the root-type is an obligate type the distribution of 

 the species is much restricted, but where it undergoes modification with 

 changed environment the distribution of the species is much less confined. 

 It is of interest to note especially that as a rule it is the latter kind of root- 

 system that is developed by such plants as occur where the soil conditions 

 are most arid, that is, on the hamada or its equivalent, and not the former, 

 from which it follows that the generalized type of root-system is really the 

 xerophytic type par excellence, and not the type with the most deeply 

 penetrating tap-root, as might be supposed. 



* Die Wasserversorgung und die osmotischen Druckverhaltnisse der Wiistenpflanzen. 

 Zeitschr. f. Botanik, 4, 191 1. 



