INTRODUCTION 



of an irregular mass of small cells, while the smaller rays, on 

 the contrary, possess much larger cells, usually in a single 

 row one above another, and are difficult to see in any section. 

 Furthermore, the large rays are frequently compound, or 

 composed of many rows of cells, and run direct from point 

 to point, whereas the small rays appear to avoid all the large 

 pores which lie in their course, or to be pushed aside by 

 them. This latter character is found with great frequency 

 in woods having rays of uniform size, whether they are 

 broad and firm, or merely like fine threads meandering amongst 

 the pores (Fig. 143). In point of size the rays may vary con- 

 siderably from the broad ones in the classes mentioned, through 

 all possible grades to the excessively fine ones of the Horse- 

 chestnut (Fig. 32). They vary greatly in the same piece of 

 wood, or in different individuals of the same species, or in the 

 wood of allied species : but like all the other characteristics, 

 there is a limit, a maximum and a minimum, fixed for each kind. 

 It often happens that the limit of length is difficult to fix unless 

 a very large piece of wood is available, as it is not unusual for 

 a ray to extend from ring to ring through thirty years' growth 

 or more before commencing to taper. The very large rays often 

 seem to dilate outwards indefinitely (Fig. 17), and in some cases 

 may do so, while others run their course in the space of two of 

 three annual rings only (Fig. 73). The Beech is a good example 

 to study, and the mere examination of the wood of a plane or 

 other carpenter's tool will generally suffice to demonstrate 

 this point. Whenever a piece of well-smoothed wood is met 

 with, it is good practice to examine it in whatever form it may 

 be made up. Something is sure to be learnt from it, providing 

 care be taken to find the transverse section where the annual 

 rings " crop out " and supply the key to the other sections. 



As the circumference of the annual rings enlarges, the original 

 rays become wider and wider apart, and new rays arise from time 

 to time in sufficient numbers to keep the proportion of the rays 

 to the mass of the wood fairly uniform. These new rays obvi- 

 ously have nothing to do with the pith, and being indistinguish- 

 able from the older ones cannot be called in any sense " Medul- 

 lary," and even those of the first year's wood have no intimate 

 connection with the pith, which may perish entirely without 

 any apparent disturbance of the function of the rays. I therefore 

 use the word " rays " without the usual qualifying adjective. 

 The term " pore " is another which needs justification as the 

 word " vessel " is considered more correct. It may be conceded 

 that it is so, yet it is less convenient and, to those who approach 

 this study without preliminary training in Botany, it is less 

 familiar and appropriate. The simple term " pore " requires 



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