The Natural System of Botany. 75 



is included in a certain order, that it must necessarily possess 

 all the characteristics of all plants of that order. Again, in the 

 case of birds, the structure and appearance of the feathers, 

 their general mark of distinction, are much varied in those 

 which approach nearest to other animals. The Cassowary, 

 for instance, which belongs to the Ostrich tribe, has feathers 

 which differ little from coarse hair, and those of the fin-like 

 win°"s of the Penguin are much like the scales offish. In like 

 manner, some of the plants belonging to the Ranunculus tribe, 

 have the parts of their flowers arranged, in threes, like Endo- 

 frenai ; yet their stems are exogenous, their leaves reticulated, 

 and they have two cotyledons. On the other hand, the Arum, 

 or Indian Turnip, has partially reticulated leaves, but an en- 

 dogenous stem and a single cotyledon. It has been already 

 mentioned that the structure and mode of increase of the stem 

 are the characters by which flowering plants are divided into 

 Exogenae and Endogenae, and these divisions are entirely na- 

 tural. There are, however, some Exogenae whose stem is not 

 marked by annual layers, and in some the predominance of 

 cellular tissue is such that they are soft and succulent, and 

 nearly resemble Cellulares. The structure of the stem, then, 

 does not offer a certain foundation for anj r division of the pri- 

 mary classes into sub-classes or orders. No characters uni- 

 form in particular families are afforded by the Roots, since the 

 modifications of their form are few, and cannot be applied 

 with certainty. One general rule is, however, that no Exo- 

 genous plant has that form of root-like stem called a bulb, nor 

 the prostrate stem which sends down roots from its lower sur- 

 face, called a rhizoma. The external figure of the stem is 

 sometimes one of the distinctions of an order. In Menisper- 

 meae, the Cocculus tribe, the stem is always twining ; in Labi- 

 atae, the Mint or Catnip tribe, the stem is always square, and 

 in one or two other orders its angular or cylindrical figure is a 

 mark of difference. The leaves are of the greatest use in 

 affording characters for determining the relations of plants. 

 One of the most important of these characters is the relative 

 position of the leaves, though this may be of much greater 

 value in some orders than in others. In Gentianeae, the Gen- 

 tian tribe, and in Labiatae, they are uniformly opposite, so that 



