!•] 



MEDULLARY RAYS. 



33 



of surface ; and when so used in decks, flooring, etc., 

 rendering it dangerous to walk upon. 



In the transverse section of a tree will be found a 

 number of lines radiating from the centre and presenting 

 a star-like appearance. These are the medullary rays 

 of the botanists, but are best known to carpenters as the 

 silver grain, or felt {a, Fig. 3). This peculiarity of 

 appearance is due to thin plates of compressed cellular 

 tissues, which usually run continuously from the pith 

 to the bark. In 



some timbers „, p 



{e.g. Oak, Beech, 

 etc.) other series 

 of medullary 

 rays are found, 

 overlapping or 

 scarfing by, but 

 not touching, the 

 larger rays {b. 

 Fig. 3). If, there- 

 fore, we care- 

 fully examine 

 the smooth sur- 

 face of a transverse section of the stems of most trees, we 

 can generally trace these thin plates or rays. They can 

 be seen to great advantage in the Beech, and, more or 

 less, in all the varieties of Oak ; but they are far less dis- 

 tinct in the Lime, Yew, and Chestnut, and in the Firs 

 and Pines they cannot be obviously traced without 

 lenses, although botanists know them to be present. No 

 timber trees are devoid of medullary rays, and their 

 peculiarities of colour, size, number, etc., are of great 

 value in determining different timbers. Thus Oak and 

 Beech, etc., have two sizes of medullary rays, a few 



D 



FIG. 3. 



