I
��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 surfjicc ; for in the Great Desert the reduction of Paniceec is still more remarkable : this tribe l^einjr to Poacca), in that region, in the proportion of only five to eighteen.
Dr. Oudncy 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 ])articular notice, the first is Avena Porskalii of Vahl. The speci- mens in the herbarium wdiich weve 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 Forskal ; I have received from M. Delile specimens both of ,his A. Forslialii and arundlnacea, described and figured in his 'Flore d'Egypte'; and am also in possession of others in somewhat dift'erent states, collected in Egypt by ]\I. Nectoux and Dr. Sieber. From a comparison of all these specimens I am led to believe that A. Forskalii and arundinacea are not specifically distinct ; and it is at least evident that arundinacea more nearly approaches to the plant of Forskal than that to wdiich j\I. Delile has applied the name Forsl:alii.
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 ForsJialii would chieflj^ 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 up])er joint is produced beyond the lower, a short calcar actually exists before separation, and
�� �
