144 Mr Nicol on the Structure of 



and in such cases the transverse axis is nearly perpendicular to 

 the longitudinal vessels. When the discs are very near each 

 other, the contiguous surfaces often become rectilinear, and 

 when they are large, they often project into the partitions, and 

 give to their edges an undulated form. 



These discs are composed of a considerable number of con- 

 centric lines, which<are always very much crowded together, es- 

 pecially near their circumferences. A space at the centre often 

 contains one circular curve surrounded with a broadish curve, 

 which in some kinds of wood has an elliptical form. The curves 

 or lines towards the centre are often not exactly in the same 

 plane as those towards the outer edge, their focal distances be- 

 ing somewhat different. But in some pines the whole disc dis- 

 plays a pretty uniform distribution of curves from the circum- 

 ference to the centre. It is impossible in words to convey to 

 the mind any idea of the beautiful appearance of these discs, 

 when duly illuminated and viewed with a single lens of the re- 

 quisite magnifying power. Fig. 2, Pi. II., will convey some 

 idea of their structure as seen in the Pin us strobus from North 

 America; but the minute assemblages of prismatic colours as 

 seen in the microscope, cannot be represented either by the 

 pencil or the tool of the engraver. 



In some of the Coniferae the discs are arranged in the vessels 

 in single rows only, in others they are arranged in double as 

 well as in single rows. In Pinus sylvestris, in P. Abies as well as 

 in the Juni})ers, Thujas, and Cypresses, I have not observed 

 more than single rows of discs in each vessel; but in Pinus stro- 

 bus, P. canadensis, Taxodium disticha, and in the Araucarias 

 from New Holland and Brazil, double as well as single rows 

 are often to be seen. When double rows of discs occur in one 

 vessel in Pinus strobus, P. canadensis, or in Taxodium disticha, 

 the discs of both rows are placed in such a manner that a line 

 drawn from the centre to the surface of the wood would pass 

 through the centres of each, or in other words, the discs in one 

 vessel are placed side by side, and never alternate with each 

 other. This arrangement will at once be seen by inspecting the 

 figures in Plates II. and III., and it may be right to bear it 

 in remembrance, as a very different arrangement will be shown 

 to exist in the double rows of discs in the Araucarias. There 



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