INTRODUCTION 7 



according to what is known as the Natural System, 

 an endeavour to group plants according to such 

 external characters as those which we have here 

 briefly indicated. There is another set of less 

 obvious distinctive features which we are able to 

 illustrate fully for the first time: the microscopic 

 structure of the woods of the different species. The 

 transverse sections here represented — sections, that 

 is, cut. at right angles to the grain and axis of growth 

 of the woods — have been -cut so thin as to be trans- 

 parent, and have been magnified ten or, in niost cases, 

 thirty diameters. Each photomicrograph here repro- 

 duced has been taken from a section of wood five-eighths 

 of an inch square; but, as "thirty diameters" means 

 that any linear dimension of the object is enlarged 

 thirty times, it is obvious that by no means the whole 

 of these small square sections can be included on one 

 of our pages. A casual glance at this remarkable 

 series of illustrations will show that, with consider- 

 able uniformity, they present also much individual 

 variation. 



The woods of all our famiUar trees, growing as 

 they do in a land subject to marked seasonal change, 

 exhibit annual rings of growth. From the number, 

 width, and concentric regularity of these rings we 

 may read not only the age of stem or branch in 

 years, but also something of the climatic fluctuations 

 of those years, and something as to the surround- 

 ings of the tree. The newer, outer rings often diiTer 

 markedly in colour from the older ones nearer the 

 centre ; and, with this difference in colour, there is 

 generally a difference in strength and durability. 



