there are larger or smaller numbers of annuals, but for a short time only. Areas with less 

 than 100 mm. annual rainfall are to be designated as desert, and only on sand and in the 

 wadis is there somewhat denser vegetation. Under still more arid conditions, only in 

 wadis and oases do we find larger numbers of individuals in what can be called an as- 

 sociation. 



Such is the natural picture where man has not interfered. This however is seldom 

 the case. The largest part of the border forest belt, has been converted to steppe, the 

 stQJpes to semi-desert, and semi-deserts into deserts. In the Orient, where over- 

 grazing has been the practice for millenia, and where the equilibrium between plant, ani- 

 mal and man has long been disturbed, this development is especially prominent. In or- 

 der to create a regeneration here, we must first determine the climax association of the 

 region under consideration. 



The reconstruction of the original climax associations, and the mapping thereof, is 

 one of the most important tasks of the plant ecologist in these regions. Without this, 

 planning land- use cannot be on a sound basis. Another line of research is based on the 

 biological rxiles of climatic extremes. At a boundary of the distribution of a species or 

 of an association, the smallest variations on the environmental conditions are decisive 

 for its existence. Plant ecologists must therefore determine these ecological ampli- 

 tudes and the geographical boundaries on the one hand, and, learn to recognize the vari- 

 ations in the environment, and to measure them quantitatively on the other. Comparison 

 between the two phenomena will always yield most important and far reaching results. 



Furthermore, when we wish to create a more dense plant cover, or to increase the 

 population density of a particular species, we must also take into account shifts in the 

 complex of factors in these boundary regions. I wish to present the example of the 'IE- 

 factor', the factor complex involving insolation and exposure. In the low latitudes, it is 

 surprising what large differences in micro- climate result from variations in the degree 

 of inclination on the same slope. Ashbel found that the difference in insolation between 

 a horizontal surface at the latitude of Jerusalem, and a 40° north facing slope is as 

 great as that between two horizontal surfaces, one located in Jerusalem, and the other 

 in Paris. Testing the results of my own investigations with regard to the decisive ef- 

 fect on vegetation, the same e^^erience has been made in U.S.A., Australia, etc. As a 

 result, we can pass through three distinct floral regions on a northern slope in the Wild- 

 erness of Judea east of Jerusalem, all within a few metres of each other. On a 25° slope 

 we find a Mediterranean flora, on horizontal surfaces a Saharo- Sindian, and on inter- 

 mediate slopes we find a transitional Irano - Turanian flora. 



The whole Far Negev, (that is the part of Israel lying south of Beersheba), can be 

 considered to be an especially favourable area for experiments on all arid regions and 

 therefore my department there has set up three permanent observation stations, which 

 are complementary to each other both edaphically and climatically. One is situated in 

 the mountains near the ruins of the ancient city of Abdeh. Here we are attacking pro- 

 blems of pasture regeneration and the utilization under control of the torrential winter 

 floods. The second is at the oasis of Ein Ghadian in Wadi Araba, the deep trench bet- 

 ween the Dead Sea and the Red Sea; and the third is the most important Desert Garden 

 at Elath on the Red Sea, where my wife Dr Elizabeth Boyko is in charge. 



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