STRUCTURE.] QUEKETT SPIRAL VESSELS. 67 



rays. Wherever the markings occur, the saucer-shaped 

 depression is thick at the circumference, and for some 

 distance towards .the centre, but in the centre itself there is 

 a spot so extremely thin and minute, that the light, which 

 has to pass through it, becomes decomposed, and the spot 

 looks either green or red, according to the adjustment of the 

 focus. 



" Having received from Professor Bailey a specimen of fossil 

 wood which was found at Fredericsberg, in Virginia, I per- 

 ceived, on submitting it to the microscope, that it would 

 easily break into minute fragments in the direction of the 

 woody fibres, which, when carefully viewed, presented a most 

 beautiful example of casts of woody tissue, with numerous 

 spirals traversing the interior. At various points were arranged 

 the ordinary coniferous dots, and to the outside there adhered 

 small bodies of the same size, which projected beyond the 

 outline of the fibre when seen obliquely, each bearing the 

 precise representation of the coniferous disc. In other parts 

 of the field of view were some of the same bodies detached 

 from the sides of the fibres, which left no doubt that they 

 were casts of the cavities existing in the original plant, and 

 proved the correctness of the view above stated, respecting 

 the nature of these minute circular markings. Besides these 

 siliceous bodies in the fragments of the fossil, there were 

 others of such a shape as to leave no doubt that they were 

 casts of the interspaces between the cells or woody fibres/' 



Pleurenchym constitutes a considerable proportion of the 

 ligneous part of all plants ; it is abundant in liber, and forms 

 the principal part of the veins of leaves, to which it gives 

 stiffness and tenacity. 



SECT. IV. Of Vascular Tissue, or Trachenchym. 



This consists of simple membranous tubes tapering to each 

 end, but often terminating abruptly, and having a fibre gener- 

 ated spirally in the inside. 



Such appears to me to be the most accurate mode of 

 describing this kind of tissue, upon the exact nature of which 

 anatomists have, however, been much divided in opinion ; 



