The Rev. P. Keith on the Internal Structure of Plants. 119 



is divided by a transverse section, it will be found to consist 

 of an epidermis, containing a hollow cylinder of pulp, thickly 

 set with bundles of longitudinal fibres disposed in a circu- 

 lar row, and lined with a soft and spongy pith, which is it- 

 self tubular, and lined with a fine and transparent mem- 

 brane, consisting of a most intricate plexus of soft and de- 

 licate fibres, forming an ultimate cavity, which is sometimes 

 partly filled up with a fine and cottony down, or with fine 

 and transverse diaphragms resembling cobwebs. 



It should be added that the tubular stem does not neces- 

 sarily form one single and continued cylinder, except in the 

 Agarics, but rather a succession of individual cylinders united 

 to one another by joints, or knots, that form transverse dia- 

 phragms interrupting the continuity of the tube, even though 

 it is furnished with no pith, as in the grasses. But though 

 the trunk of this order of plants is often tubular, yet the root 

 is not often tubular, — though the root of Water-Hemlock fur- 

 nishes an exception, — but is wholly filled up with pulp and 

 interspersed fibre, or with layers similar to those of the orders 

 that follow. 



The third division comprehends the highest orders of ve- 

 getables, that is, orders exhibiting the highest degree of vege- 

 table organization, the caudex being now more decidedly 

 vascular in its structure, as well as more perplexingly intri- 

 cate in its analysis, as consisting of an outer, an intermediate, 

 and a central part, or, in other words, of a bark, wood, and 

 pith, each having an aspect and texture peculiar to itself. 



It has been observed that the progressions of nature are 

 not made per saltum, and doubtless there is truth in the re- 

 mark. The various tribes of plants graduate imperceptibly 

 into one another ; and if the arrangements of nature inter- 

 mingle, so must ours also. Thus, between the first and second 

 of the foregoing divisions there are plants to be found par- 

 taking of the character of both, as the dissection of the lobes 

 and peduncle of Marchantia polymorpha will evidently show. 

 The lobes are cellular, the peduncle is vascular*. The same 

 thing may be said of the second division, and the division 

 now under our consideration (as coming in sequence). The 

 structure of this last is best exemplified in shrubs and trees. 

 Yet nature does not pass per saltum from plants that are purely 

 herbaceous on the one hand, to plants that are purely woody 

 on the other. There is an intermediate order, partaking of 

 the character of either class, that forms the connecting link. 

 In the latter case the wood is perfect, in the former case it is 

 imperfect. 



* Keith's Phys. Bot., vol. i. p. 228-344. 



