38 



THE ELEMENTARY STRUCTURE OF PLANTS. 



upon what are called dotted ducts ; as in Fig. 32, b, and Fig. 56, 57. 

 All markings of tliis kind are thin spots, which, for some rea^^on, 

 have not partaken in the general thickening of the Avail. Although 

 they are not primarily pores or i-eal perforations, yet they often be- 

 come so with age, by the destruction of the thin primary membrane, 

 after the cell has lost its vitality. Fig. 82 shows these dots on the 

 wood-cells and the ducts of the Plane-tree. And Fig. 33, represents 

 ing some of the wood-cells more highly magnified, explains their 

 real nature, namely, as deep pits in the thick wall. It will be seen 

 that the pits of contiguous cells exactly correspond ; showing that 

 there is nothing accidental in the origin or the arrangement of these 

 markings. They are manifestly designed for maintaining communi- 

 cation between contiguous cells, and for the ready conveyance of the 

 sap from cell to cell, notwithstanding the thickening of their walls. 

 Of similar nature, although of greater size, are the so-called 



46. Discs or Circnlar Markings of Coniferous Wood (Fig. 34-37).. 



These are of universal occui-ence in the wood of Pines, Firs, and all 

 that family of Coniferous trees ; and something very like them, if 

 not the same, occurs in the Winter's-Bark tree (as long ago shown 

 by Mr. Brown), th(_' Star-Anise, and even in the Magnolia, and other 

 aromatic trees. They may readily be seen in a thin Pine shaving, 

 taken parallel with the silver-grain : for in the Pine family they are 

 nearly all found on the lateral walls of the cells, few or none being 

 Adsible on the sides which look towards the bark or towards the 



FIG. 34. Piece of a Pine stiaTing, magnified, to show the discs or thin spots which appear 

 on the cells of all Coniferous wood. 35. A separate cell of the above, more strongly magnified. 



]?IG. 36. A small portion of five cells of White-Pine wood magnified ; seen both in trans- 

 verse and longitudinal section, a, a, discs, in tmnsverse section : &, &, discs as looked down 

 upon in longitudinal view. 



FIG. .37. A highly magnified transverse section of one complete wood-cell, connected with 

 adjacent cells, and of a disc (a) : after Mohl. 



