1883.J '^Ol [Rothrock. 



season, and that whether much or little wood is subsequently formed the 

 number of the ducts will not greatly vary. Hence, then, for White Oak we 

 may assert that the specimen with the larger year's growth is, other things 

 being equal, the better. Very frequently two duct cavities are thrown into 

 one, so that the width is greatly increased. These may usually be distin- 

 guished from true ducts by the Irregular and disintegrated walls, which 

 serve to explain the process by which the size was attained.* The above 

 rule, as to the relation between size of "year's growth " and value, in Oak 

 I have made the subject of some investigation, taking as test cases speci- 

 mens of timber upon whose value opinions had been given by the most 

 competent workers in the wood. 



Hickory, good and bad (certainly Carya alba and C. porcina), involves 

 another element than mere size of the annual ring. Though I must here 

 add that the best bit of C. porcina I have ever seen was also one that had 

 the largest year's growth I had ever seen. In this wood (Hickory), the 

 large ducts are not so clearly limited in their production to the early part 

 of the season (especially if the stick be one of poor quality), but are, or may 

 be, clearly scattered through the wood. And the quality of the wood is de- 

 termined mainly by the number and size of these ducts. Thus in bad Pig- 

 Nut Hickory (C. porcina) I find in a surface of a quarter of an inch square, 

 sixty -five, each with an average size of .01428 inch ; as against twenty-seven 

 ducts having an average width of .01224 inch in good Hickory of the same 

 species. 



To a greater or less extent the same statements, as to cause of difference 

 between good and bad qualities of Chestnut, and Locust (Robinia pseud- 

 acacia), will apply. 



Figure C. illustrates the marked tendency which the ducts have to be as- 

 sociated in Hickory. It also shows the elfect of the growth in pushing aside 

 one of the medullary rays, 9 b. It is not uncommon, however, in this wood 

 to find these rays broken by the growth of the.duct, and in Oak this is still 

 less rare. I have frequently seen specimens of bad White Oak which were 

 as porous as the average Red Oak, the ducts being, as shown by the micro- 

 meter, quite as great in their diameter. 



The medullary rays or "silver grain" appear also to have important re- 

 lation to value of Oak certainly, and probably of Hickory, to say nothing 

 of other kinds of timber. The fibres and ducts are ordinarily characterized 

 as the vertical system from the line in which they are elongated. With 

 equal propriety then the medullary rays are spoken of as the horizontal 

 system of the plant, because they are elongated at right angles to the fibres 

 and ducts. From the thick walls of the cells constituting these rays, we 

 might suppose they had to do with the lateral strength of the timber. This 

 view is partially confirmed by a microscopic examination of the cross sec- 

 tion of the difierent woods ; as upon the whole. Red-wood, Chestnut and 

 White Pine show either that these rays are fewer in number or less strongly 



* Vei"y often this process of disintegration of the wall may convei't a true 

 duct into a mere cavity without walls. 



