154 Botanical Society of Edinbitryh. 



the yellow rings seen in the transverse section are the ends of those 

 tnhes— and that the red colour of the tubes, like blood seen in the 

 veins of the Frog's foot, is an aggregation of the yellow bodies 

 strewed over the section like blood-cells in single file. 



" For a moment let us reflect upon the appearance presented by a 

 cell or tube, as seen on the field of the microscope. In order that 

 either of these bodies may be seen by transmitted light, a certain 

 amount of transparency is necessary ; the slightest curvature or undu- 

 lation on the surface causes the light thrown from the reflector to be 

 diverted from the eye of the observer, and the part so curved appears 

 dark ; this is invariably the case at the margins : the outlines of such 

 bodies are distinctly defined and black, whatever may be their colour 

 or form, and this blackness is diffused inwards according to their 

 rotundity. 



" How is it, then, that histologists have in this case departed from 

 an invariable optical law, and have described appearances void of out- 

 line as cells and tubes ? But even admitting them to be so, I would 

 then ask, why have these not been carbonized like the adjacent struc- 

 tures which form the black mass ? Wanting these two characteristics, 

 no one need hesitate to say that they are neither cells nor tubes. 



" With regard to the red colour being an aggregation of the yel- 

 low, it is enough simply to state that that is impossible, from two 

 facts — the section is reduced to an equal thickness, and the yellow 

 bodies occur within the red spaces, their definition being then as 

 complete as wheu imbedded in their usual black matrix. 



" I have said that Mr. Quekett expresses a notion similar to mine, 

 but again returns to the usually received opinion ; here are his 

 words : — ' If a small cubical block of any kind of coal be examined 

 under a power of fifty diameters, four of its six sides will exhibit more 

 or less of a fibrous structure, precisely like that of wood' 



" Now, if a block of wood and a block of coal be examined together, 

 with the longitudinal striae of each laid in the horizontal direction, 

 they will present very different appearances. In the case of the wood 

 it will be seen that only two of its sides, which I shall call the back 

 and front, present the striated appearance ; in the other two the ends 

 are distinctly transverse sections, and show the ends of organisms. 

 But this is not the case with a block of coal, which may be likened 

 to a pack of cards, the four sides in question having a similar appear- 

 ance, and, so far as structure is concerned, having each an equal title 

 to be called a longitudinal section. Where, then, is the transverse 

 section of coal, and where are the yellow rings ? 



" Hitherto our attention has been directed to the sides of the block, 

 let us now examine the top and bottom, or horizontal surfaces : there 

 the rings are found, and being there, cannot be the ends of the sup- 

 posed tubes, nor have any relation to them, seeing that their longi- 

 tudinal direction is that of the surface now under examination. 



" The grinding and polishing process is very instructive. On the 

 sides of the blocks many series of rings may be ground off without 

 much alteration in the appearance of the strise, while in reducing the 

 horizontal surface the transparent parts are constantly changing their 

 form. The first evidence of transparency which presents itself is a 



