OF CENTRAL AFRICA. 301 



been expected, in the climates in which the collection was 

 formed ; it seems, however, to be connected with the 

 nature of the surface ; for in the Great Desert the reduction 

 of Panicese is still more remarkable ; this tribe being 

 to Poaceae, in that region, in the proportion of only five to 

 eighteen. 



Dr. Oudney remarks, with respect to the grasses of the 

 desert, that he observed no species with creeping roots; 

 for a species of Arundo related to Phragmites, which 

 he notices as the only exception, is not properly a desert 

 plant. 



Among the very few Graminese deserving particular 

 notice, the first is Avena Porskalii of Vahl. The speci- 

 mens in the herbarium which were collected in the Desert 

 of Tintuma in some respects differ from all the others that I 

 have seen of this variable species. In the Banksian her- 

 barium there is an authentic specimen from Porskal ; I 

 have received from M. Delile specimens both of |his J. 

 ForsJcalii and arundinacea, described and figiKed in his 

 ' Plore d'Egypte'; and am also in possession of others in 

 somewhat different states, collected in Egypt by M. Nectoux 

 and Dr. Sieber. From a comparison of all these specimens 

 I am led to believe that A. Porskalii and arundinacea 

 are not specifically distinct ; and it is at least evident that 

 arundinacea more nearly approaches to the plant of 

 Porskal than that to which M. Delile has applied the name 

 Forskalii. 



This grass, which does not belong to Avena, is referable 

 to Danthonia, from the structure of the outer valve of its 

 perianthium. But Danthonia requires subdivision into 

 several sections, of which, perhaps, our plant may be con- 

 sidered as forming one. 



The character of the section established on Danthonia 

 Forskalii would chiefly consist in the very remarkable obli- 

 quity of the joints of the locusta, which is, indeed, so great, 

 that after their separation each flower seems to have at the 

 base an almost vertically descendent spur ; and as the in- 

 ferior extremity of the upper joint is produced beyond the 

 lower, a short calcar actually exists before separation, and 



