84 YEEWS OP THE MICROSCOPIC WORLD. 



a thick juice of the Oak which is not sap. but bears the same relation to it in the 

 Oak as the turpentine of the Pine to the sap of that tree. The rays of cellular 

 tissue, diverging from the centre to the bark, are in this tree divided into two kinds 

 as regards size. The first are broad insertions ; which, for the most part, are of the 

 same size, and are disposed around the centre at regular intervals; the second are 

 the finer radial divisions, which in like manner are uniformly arranged and occupy 

 the spaces between those of the first class. 



In the section of the Pear, all the dividing lines are seen proceeding from the 

 centre, but in the Oak and some other trees numerous white waving lines run 

 across the radial divisions. These undulating rings constitute, in a great measure, 

 the beauty of the Oak, and are considered by Grew as sap-vessels, which once 

 existed in the bark, but in process of time became condensed and hardened into 

 wood. 



WHITE OAK. A section of the common White Oak, magnified one hundred and 

 thirty-seven times, is delineated in figure 131. The broad band a a is one of the 

 insertions of cellular tissue radiating from the pith ; it is exceedingly compact, for 

 no pores can be detected within it when subjected to this high magnifying power. 

 Narrower insertions of cellular tissue b &, &c., traverse the wood in the same direc- 

 tion in irregular waving lines. The spiral vessels d c, &c., are scattered in consider- 

 able numbers throughout the wood, occupying a large proportion of its space. 

 They vary much in size, the smallest, as d for instance, not being more than one 

 four-hundredth of an inch in diameter ; while one of the largest, as c, measures not 

 less than the eighteenth part of an inch across it. 



Two sets of these large spiral vessels are seen in the figure, which, like those be- 

 held in the section of the English Oak, are arranged along the inner margin of each 

 annual ring of wood ; the distance between the two clusters being the thickness of 

 one year's growth of wood. The wood itself appears like lace-work, being filled 

 with minute pores, varying in diameter from one twelve hundred and fiftieth part of 

 an inch to one twenty-Jive hundredth. 



