40 



THE ELEMENTARY STRUCTURE OF PLANTS. 



ing this mechanical explanation. Although they are not pores or 

 real perforations of the wall, as has been thought, and perhaps is 

 still maintained by some, yet they often become so with age, by 

 the breaking away of the thin primary membrane, after the coll 

 has lost its vitality. The subjoined dissections of the wood of the 

 American Plane-tree, already referred to, clearly show the true 

 nature of these dots, which here abound on the proper wood-cells 

 as well as the larger ducts. Except in their lesser size and great 

 er depth, arising from the more extensive thickening of the tubes 

 they do not essentially differ from the well-known 



45. Discs, or large circular dots, which mark nearly all the 

 wood-cells of the Pine Family (Fig. 23, 24). Uhese are thinner 

 spaces, which consequently appear more transparent than the rest 

 of the tube (except when filled with a film of air), when viewed 

 by transmitted light. The discs of contiguous tubes are applied di- 

 rectly to each other, face to face (just as the 

 canals or thin places of other cells thickened 

 by secondary deposits correspond, 42), and 

 each is a little depressed, so that a lenticular 

 space is left between them, as between two 

 watch-glasses put together by their circumfer- 

 ences. They are seldom found on the sides 

 of the wood-cells that look towards the bark 

 \j or towards the pith ; while they abound in a 

 section made in the direction of the lines of 



23 24 



silver-grain. The dots on the wood-cells of 

 the Plane-tree, on the contrary, are most abundant on the sides 

 that look towards the centre and the circumference of the trunk. 

 Although of universal occurrence in the Pine Family and the relat- 

 ed order Cycadacese, these discs are not restricted to them, as was 

 once supposed. Mr. Brown long since showed that the wood of 

 the Winter's-bark tree was similarly marked ; and our Fig. 33 

 represents them as they appear in the Star- Anise of Florida, which 

 belongs to the same natural group of plants. They are said to be 



ashes, Prof. Bailey, of West Point, has enabled us to detect and distinguish 

 vegetable tissues in anthracite coal. See Sittiman's Journal, Vol. I., New 

 Series. 



FIG. 23. Piece of a Pine-shaving, magnified, to show the dots or discs which appear on the 

 cells of all Coniferou3 wood. 24. A separate cell of the above, more strongly magnified. 



