INTRODUCTION. 27 



Annual rings also are very useful distinguishing 

 features in most woods. There are none at all in 

 Palms, Bamboos, Tree-ferns, etc. ; and they are indis- 

 tinguishable in many exotic timbers — e.g. many Indian 

 Oaks and other timbers — while they differ in course, 

 breadth, and sharpness in various ways in other woods. 



Heart-wood is formed by many trees, and is quite 

 different in colour, hardness, density, etc., from their 

 sap-wood ; but there are others which show no traces 

 of it to the observation. Some trees form it early, as 

 the Oak, others late, as the Ash, and great differences 

 occur in these Respects. 



There are numerous other points of structure, most 

 of them technical in character and not suited for dis- 

 cussion here, that help the expert to determine the 

 nature of a piece of wood. The resin-canals of the 

 Pines, Larches, and A nacardiacem ; the so-called " pith- 

 flecks " of many Birches, Alders, Hawthorns, Poplars, 

 etc. ; the peculiar contents of the cells in Birch, Alder, 

 Mountain Ash, Pear, Ebony, etc. ; or of the vessels in 

 Teak, Robinia, and others, are all instances. To these 

 may be added the peculiarities of weight (per cubic 

 unit), grain, hardness, toughness, and even odour — e.g. 

 Teak and Cedar. 



The botanist is also concerned with the functions 

 of timber. Broadly speaking, these are, support and 

 flexibility — the very mechanical purposes to which we 

 apply timber apart from the tree — due principally to 

 the fibres and tracheids ; the conduction of water and 

 air — properties especially attributable to the vessels, 

 and to which regard must be had in all cases of flotation, 

 etc. ; and the storage of organic materials, the substances 

 we have to take into account, owing to their putrescible 

 nature, in preserving and seasoning timber, and to 



