66 BOTANICAL FEATURES OF THE ALGERIAN SAHARA. 



which, contrary to the expectation, are apparently wholly fibrous at Bis- 

 kra, although they are in part fleshy farther south, where the soil conditions 

 are surely more arid. At Biskra, also, the only plant observed with typi- 

 cal generalized root-systems (Peganum) does not grow where the soil is 

 shallowest, but where it is relatively deep. So far as seen, also, plants 

 growing where the soil is shallow have either a generalized root-system 

 or a root-system approaching this type, even if the branching is only 

 relatively deep. 



GENERAL SUMMARY AND CONCLUSIONS. 



THE ENVIRONMENTAL CONDITIONS OF PLANTS IN ARID REGIONS. 



The environmental conditions encountered by plants in the arid regions 

 are widely different from those of moister regions. Precipitation is not 

 only slight, but it shows an enormous range in variation from year to year; 

 the rate of evaporation is high; the temperature of the air and soil varies 

 widely both during the day and with the seasons; the light is of great 

 intensity, and the soil is low in humus content and may contain an excess 

 of salts. These, the most striking physical factors of deserts, are present 

 in different combinations with resultant differences among deserts, and an 

 arid region may be so large as to include such variation within its borders. 

 Also, a desert may be so far from the ocean, or it may include such 

 diversity of topography, as to show great variation in biological features 

 as well as in its surface phenomena. 



In the flora of any arid region, the mutual relations of the constituents of 

 the flora and its general and detailed relation to the physical environment 

 are quite different in the main from these features in the flora of the more 

 humid regions. Thus, the leading relation is the relation to water, and on 

 the response of the plants to this relation much of the phenomena associ- 

 ated with plant life in the desert directly depends. For example, in the 

 extreme deserts it is probable that the elements of competition between 

 the perennials, which is an important factor in the survival of a species 

 in the moister regions, is wholly lacking. It should be noted, however, 

 that in the less extreme deserts, as in the vicinity of the Desert Laboratory, 

 competition exists between plants, although this is not at first apparent. 

 In this case the competition is not for room, but for water, and is not mani- 

 fest by palpable crowding, but by invisible competition of the roots. Thus 

 the reactions are with the physical environment and are exhibited in a 

 variety of ways, some of which concern the plants themselves in an inti- 

 mate manner, being morphological and physiological, some being concerned 

 with the flora as a whole. The environmental responses are often obscure 

 and complex, but in other instances they are less obscure and apparently 

 direct. 



