PREFACE 



THIS GUIDE aims at giving information on certain points which are not 

 easily accessible elsewhere or have hitherto been treated in a manner 

 unsuitable to our purpose. It is therefore assumed that the reader has 

 sufficient knowledge of the elementary structure of wood, to follow the 

 descriptions. Two objects are kept in view: firstly the observation of 

 the characters of the different species, and secondly the utilization of 

 those characters to discriminate between one species and another. 



No so-called "specific gravities" are given, since those so far recorded 

 are taken either from long series of well-worked species which have so 

 wide a range as to be quite useless for comparison (for instance between 

 0-45 and 0-90 for the Scots Fir), or from short series of less familiar 

 woods (say 0-70 and 0-75) ; the latter, in spite of appearing to vary but 

 little, may average the same as the former. Hence the rather vague terms 

 "heavy," "moderately heavy," and so on are used, since at least they 

 do not mislead. 



The illustrations are restricted to figures (taken from photographs 

 of sections in the Cambridge Research laboratory) which are deemed 

 necessary where the critical detail can be seen only by means of the 

 microscope or where such detail is a difference of degree and not of kind. 



The Generic numbers attached to the species are those of Bentham 

 and Hooker's Genera Plantarum, according to which system, the speci- 

 mens of wood in the collection of the School of Forestry are arranged. 

 Any specimen needed for comparison can be found immediately by 

 means of these numbers. 



Every effort has been made to find points of difference in kind, those 

 of degree, such as the varying size or numbers of the rays and vessels, etc., 

 or the colour, hardness, weight, etc. not being reliable. This has been 

 done satisfactorily in all cases except those of the Poplars, Willows and 

 Horse Chestnut, a most difficult group, the Common and Wych Elms, 

 and the Mahoganies : all these cases demand a training of the senses only 

 to be acquired by the handling of many specimens. 



HERBERT STONE. 



WOOD RESEARCH LABORATORY, 

 CAMBRIDGE. 



April, 1920. 



