116 The Rev. P. Keith on the Internal Structure of Plants. 



petioles, which are divided into a prodigious number of rami- 

 fications mutually embracing and intersecting one another, 

 and forming a fabric similar to a piece of fine network. The 

 principal fibre, extending from the base to the apex of the leaf, 

 forms what is called the midrib; the lateral ramifications form 

 what are called the nerves or veins; terms borrowed from the 

 animal kingdom, but calculated to mislead as having no func- 

 tions in common with the nerves or veins of animals. Yet 

 many leaves have no transverse fibres. In monocotyledonous 

 plants the fibres are parallel to the midrib. But the most 

 singular circumstance in the structure of the leaf is, not 

 that the fibres are subdivided into a variety of ramifications 

 forming a fine network, but that the network thus formed is 

 double, being actually composed of two distinct layers, the 

 one corresponding to the upper, and the other to the un- 

 der surface of the leaf. In the leaf of the Orange-tree the 

 network consists of even three layers, as the dissection, or 

 rather the maceration, of that leaf will show, and no lan- 

 guage is able to convey an adequate idea of the delicacy and 

 intricacy of the web. 



The Caudex, or Mass of the Trunk and Root. — In opening 

 up the caudex, whether ascending or descending, the dissector 

 will soon discover that its internal structure, like its external 

 aspect, or habit, is materially different in different tribes of 

 plants. This was long ago pointed out by Grew in his Ana- 

 tomy of Trunks, and w ? ell illustrated by plates. It was further 

 illustrated by Plumier in his Treatise on the Ferns of Ame- 

 rica, as also by Linnaeus; and still more recently by Messrs. 

 Daubenton and Desfontaines*, who have investigated the sub- 

 ject with great ability, as we cannot but admit, but who by 

 generalizing their notions, perhaps somewhat too hastily, 

 have applied and restricted the modes of organization which 

 they illustrate rather incorrectly to certain tribes of plants. 

 Thus, the two modes of internal structure which they demon- 

 strate and describe are presumed to correspond respectively 

 to monocotyledonous plants on the one hand, and to dicoty- 

 ledonous plants on the other, the caudex of the latter be- 

 ing represented as composed of distinct concentric and diver- 

 gent layers, and the caudex of the former as exhibiting merely 

 bundles or assemblages of large, longitudinal, and woody 

 fibres interspersed throughout a pith,— but the fact is, that the 

 two modes of internal structure here specified do not uni- 

 formly and respectively pervade the two grand divisions of 

 plants now in question. If all monocotyledonous plants are 



* Mem. dc /' Instil. Nat., torn. i. 



