The Rev. P. Keith on the Internal Structure of Plants. 187 



denominated the alburnum, till in the process of vegetation it 

 becomes an inner layer, in its turn, more solid and more con- 

 densed, and is ultimately converted into perfect wood. 



The divergent layers, which intersect the concentric layers 

 in a transverse direction, constitute also a considerable pro- 

 portion of the wood, as may be seen on a horizontal section of 

 the stem of the Fir or Birch-tree, on which they present an 

 appearance like that of the radii of a circle. But if the wood 

 is split longitudinally, fragments of the divergent layers will 

 be seen adhering to the surface of the fracture, in the form 

 of large and smooth plates, which interweaving themselves 

 among the concentric layers, in the manner of an irregular 

 wickerwork, form a sort of tense binding, that cements and 

 unites the whole. This appearance is peculiarly conspicuous 

 on the riven surface of the Elm-tree or of the Oak. 



In following up the analysis of the ligneous layers you 

 cannot separate the two sets, so as to exhibit each of them 

 entire ; but as the divergent layers are soluble in certain fluids 

 in which the concentric layers are not soluble, the latter 

 may be exhibited pretty entire by means of the destruction of 

 the former. That which seems at first sight to be merely 

 an individual layer, proves upon further inspection to be a 

 group consisting of component layers, finer and smaller still, 

 till at last you can follow the division no further. Du Hamel 

 macerated a piece of the trunk of an Oak-tree in water, with 

 a view to dissolve the divergent layers, and found, after a 

 long time, that the minuter divisions of the concentric layers 

 consisted ultimately of an assemblage of longitudinal or woody 

 fibres, so as to form a network similar to that of the liber. 

 The root of the Artichoke, after being pulled up out of the 

 soil, and exposed for a considerable time to the action of the 

 atmosphere, affords, perhaps, the most beautiful of all exam- 

 ples of this sort. It separates, thus, into thousands of layers 

 of network, each as fine and as delicate as a piece of Brussels 

 lace. 



Yet this mode of analysis gives us no knowledge of the 

 structure of the divergent layers. We must consequently 

 have recourse to the aid of the microscope. Take a minute 

 slice of a divergent layer from the riven surface of an Oak or 

 Elm, and put it under a good glass, and you will find that it 

 has the appearance of being composed of an assemblage of 

 parallel fibres, or threads of contiguous vesicles not forming 

 a network, but closely crowded together, and compressed into 

 a thin plate. It is apparently nothing more than the vesicles 

 or cellular tissue of the pulp that existed originally in the 

 alburnum, now deprived of its parenchyma, or contained fluid, 



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