148 Mr Nicol on the Structure of 



riae from any other tribe of Coniferoe. In the Araucarian rows, 

 the discs are always placed near one another and at equal dis- 

 tances, and each row, whatever be its extent, terminates abruptly 

 at both its extremities. In some of the single rows the discs, 

 though near one another, are quite distinct and pretty regular 

 circles; but it sometimes happens that the discs areas if squeezed 

 together, when they become slightly elliptical. 



In the Araucarias the discs, particularly those in the single 

 rows, are larger and much more distinct than in the yew, but 

 they are smaller and more uniform in sizie than in any of the 

 true pines. The concentric circles of which they consist are 

 often very distinct, and, when duly magnified, they present a very 

 beautiful appearance ; but by far the most striking feature of 

 the Araucarian discs is to be seen in the double rows. In the 

 pines the discs in all the double rows, as already observed, are- 

 always placed side by side ; whereas, in the Araucarias, the discs 

 in the double rows always alternate with each other. This al- 

 ternation of the discs in the double rows, has modified in a very 

 striking manner the contiguous parts of the circumference oi 

 each disc. Instead of being circular, the approximating parts 

 have become rectilinear, which has rendered each disc partially 

 polygonal, and wherever the discs in one row are equally near 

 one another as those in the two rows, their contiguous parts 

 are often also distinctly rectilinear, so that each disc is in four- 

 sixths of its circumference rectilinear, two-sixths only retaining 

 the circular form. The discs in the double rows of the Araucariae 

 are, therefore, partly circular and partly polygonal, the poly- 

 gonal part consisting of four distinct sides. 



The polygonal form which these discs have partially assumed, 

 affords a striking instance of the effects of opposite forces mu- 

 tually acting against each other in the process of vegetation ; 

 and there can be very little doubt, that were triple or quad- 

 ruple rows of discs to occur in any tree arranged in the manner 

 of those in the double rows of the Araucaria, we should have a 

 series of regular hexagons, more distinctly defined than any of 

 those hitherto seen in the fossil Coniferae. Whether such an ar- 

 rangement exists in any living tree cannot be determined, until 

 the structure of all the Coniferae be examined. That a struc- 

 ture similar to that above described exists in all the Araucariae 



