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Natural Orders.] structure of these genera may be proved by a series of species connecting them with each other, and Panicum with Paspalum.

Paniceæ have their maximum within the tropics, and they cease to exist in the most northern parts of Europe and the higher southern latitudes. Of this tribe, 99 species have been observed in Terra Australia, 79 of which were found within the tropic, and of these, 66 only within it. There is no Australian genus of this tribe; Neurachne and Hemarthria excepted, which is not chiefly intratropical.

The second tribe, which may be called, is more numerous than Paniceæ, and comprehends the greater part of the European genera, as well as certain less extensive genera peculiar to the æquinoctial countries; it extends also to the highest latitudes in which Phænogamous plants have been found, but its maximum appears to be in the temperate climates considerably beyond the tropics. The Locusta in this tribe may consist of one, of two, or of many flowers, and the two flowered genera are distinguished from Paniceæ by the outer or lower flower being always perfect; the tendency to imperfection in the locusta existing in opposite directions in the two tribes. In conformity with this tendency in Poaceæ, the outer valve of the perianthium in the single flowered genera is placed within that of the gluma, and in the many flowered locusta the upper flowers are frequently imperfect. There are, however, some exceptions to this order of suppression, especially in Arundo Phragmitis, Campulosus, and some other genera, in which the outer flower is also imperfect, but as all of these have more than two flowers in their locusta, they are still readily distinguished from Paniceæ.

In Terra Australis the Poaceæ amount to 115 species, of which 69 were observed beyond the tropic and of these 63 only beyond it; but of the 52 species that occur within the tropics 49 belong to genera which are either entirely or chiefly intratropical, and of the remaining three species, two, namely, Arundo Phragmitis, and Agrostis virginica, are very general and also aquatic plants. The distribution of this tribe, therefore, in Terra Australis agrees with that which obtains in other parts of the world.

FILICES. Of this order nearly 1000 species are described in the