GENERAL SUMMARY AND CONCLUSIONS. 8 1 



tions are the immediate result of the small rainfall. The reason for the 

 large number of plants in certain areas, as above noted, lies in their small 

 size, since it would probably be difficult to find an equal number on this 

 area were the plants as large, for example, as in southern Arizona. The 

 fact that the perennials are inconspicuous is in part because they are small 

 and in part because the leaves are either absent or greatly reduced. 



When viewed somewhat more closely, one finds other features in which 

 the flora of southern Arizona and of southern Algeria are unlike. Travelers, 

 botanical as well as non-botanical, have described the armed condition of 

 the Saharan plants until the impression is general that such plants as per- 

 sist from season to season are usually well provided with spines. What may 

 be the proportion of armed to unarmed plants in the northern Sahara I 

 do not know, but to a person familiar with the plants of southern Arizona, 

 where spinose forms are very numerous, the Algerian plants do not appear 

 especially well protected. As this appears to be a general condition, it is 

 scarcely an accident that the spines of the American species of the genus 

 Zizyphus, for example, are much better developed than are those of the 

 Algerian representative of the same genus. From the circumstance that 

 grazing by wild as well as by domestic animals is very destructive in Algeria, 

 apparently more so than in Arizona, where the results of grazing are scarcely 

 to be noted, the general facts regarding spininess in plants, as given above, 

 suggest the really small influence such animals play in shaping such a 

 character in desert plants. 



Finally, it need only be remarked that plants with a water-balance are 

 wanting in southern Algeria, and that they constitute one of the striking 

 features of the flora of the southwestern United States. 



