RESINOUS TRACHEIDS 53 



The mean ratio for all these specimens is i : 2.21, which very 

 closely approximates to that for P. macrocarpa (i : 2.41), and it 

 shows that the spring tracheid is normally twice the volume of 

 the summer tracheid. But such a ratio is subject to important 

 exceptions within the limits of the individual. Thus the values 

 for i and 2 relate to regional differences in the same specimen, 

 and the same is also true of 5 and 6, and 9 and 10. It is there- 

 fore obvious that such variations are in no sense of specific value 

 for diagnostic purposes, since they are often as widely divergent 

 within the limits of the species as between different species, 

 being determined by peculiar conditions of growth. Inasmuch as 

 variations in the relative volume of the tracheids is a feature of 

 the density of the wood as a whole, it may be supposed to bear 

 a certain relation to the strength of material and so to the value 

 of the wood for constructive purposes, but this has been shown 

 not to be the case (52, 39). 



RESINOUS TRACHEIDS 



In Araucaria excelsa a transverse section shows more or less 

 numerous elements containing resin. These are not to be dis- 

 tinguished in their general structure from the surrounding tra- 

 cheids, and they are to be recognized solely by their contents, 

 which are usually somewhat prominent. Their distribution is 

 characteristic. They occur in small, scattered groups, or more 

 commonly in rows one or two elements wide, parallel with the 

 medullary rays and in immediate contact with them on each 

 side. When the plane of section passes near the position of 

 apparently terminal walls, the latter are cut through in various 

 ways, but they never exhibit any structural features, and they 

 are therefore in no way comparable with the terminal walls of 

 the wood -parenchyma cells. In a radial section these elements 

 are seen to be long and fusiform, exactly resembling the wood 

 tracheids except for reddish-brown, transverse plates which occur 

 either close to or exactly opposite a medullary ray, --a posi- 

 tion which is more clearly seen in a tangential section (fig. 4). 



