DESERT FLORA. 147 



botanists) giving rise, this highly modified flora may yet conquer 

 still further the adverse environments with which it has to contend. We 

 have in these Desert regions the two main conditions for the evolution of 

 species according to received doctrines : (i) Abundance of unoccupied 

 space, and (2) Recent change in conditions of climate, involving variation 

 and a struggle for existence. Besides an extreme dryness of climate little 

 suited to vegetable growth, the vast numbers of rodents in the Desert must 

 operate largely to check an increase of plant-life ; and it is in accordance 

 with modern views to suppose that the excessively obnoxious qualities of 

 smell, viscidity and prickliness so commonly met with may here be of real 

 protective value to their owners. A species which jerboas and sand-rats 

 found preferable to all others would speedily disappear. Those forms 

 which are ' in harmony with their environment ' have, however, abundant 

 room to make use of, and it is often a matter of surprise a particular spot 

 is selected by a plant when areas with precisely similar conditions which 

 might be similarly occupied are devoid of growth. 



With regard to the qualities above alluded to I may be here allowed a 

 few words, although this subject has no doubt been often gone into. 

 Dr. Anderson, in his excellent ' Florula Adenensis,' already referred to, 

 has left little to be told. The more numerous flora of Sinai gives, how- 

 ever, wider groups of examples. One speedily comes to the conclusion 

 that a Desert plant will be either (i) Hoary or white, with woolliness, as 

 Morettia, Glinus, Farsetia, Reaumuria, Tribulus, Dianthus, Lotononis, 

 Neurada, Anvillaea, Odontospermum, Fulicaria, Verbascum, Stachys, 

 Teucrium, -^rua; (2) Glaucous, as Moricandia, Zilla, Nitraria, Capparis, 

 Microrhynchus, Pyrethrum ; (3) Viscid, or gummy, as Cleome, Nepeta, 

 Fulicaria, Acacia, Alhagi, Hyoscyamus ; (4) Hooking, or slicking wilh 

 hairs, as Trichodesma, Cleome, Andrachne, Forskahlea ; (5) Prickly or 

 spiny, as Zilla, Capparis, Nitraria, Acacia, Alhagi, Astragalus, Microrhyn- 

 chus, Sporobolus, Iphiona ; (6) Strongly aromatic, as Deverra, Pyrethrum, 

 Achillea, Varthamia, and many labiates ; (7) Nauseating, as Peganum, 

 Haplophyllum (Ruta), Cleome, Odontospermum ; (8) Whitened, as if 

 calcified (usually stems), as Scrophularia (edges of leaves). Astragalus, 

 Microrhynchus, Zollikoferia, Calligonum, Ephedra, Tamarix, Salsola ; 

 (9) Succulent, as Nitraria, Salsolee, Zilla, Moricandia, Boucerosia, Reau- 

 muria. These qualities are often combined in various degrees, and all the 



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