66 VIEWS OF THE MICROSCOPIC WORLD. 



year is comprised within the space K L M N; while that of the third year is 

 represented by the space M N E F. The size of the pith, E F G, is small com- 

 pared with that of the wood ; but its cells a-re much larger that those belonging 

 to the pith of the Holly. The small figure at the bottom of the plate repre- 

 sents the large section of its natural size. 



ENGLISH OAK. A section of Oak of three years' growth is exhibited in draw- 

 ing 130, varying in some respects from the several kinds of woods which have 

 already been described. It will be readily seen that the vessels of the bark are 

 here arranged in different ways. Through the middle of the bark an unbroken 

 arched baud of vessels extends from K to I, while a row of large oval clusters, 

 standing at equal distances from each other, stretch from K to L, next to the 

 wood. The vessels of this inner row are of a peculiar nature, and are termed 

 by Grew resiniferous vessels, since they contain 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 divisions of the annual rings of wood are 

 indicated by the positions of the arched lines of larger vessels, extending from 

 M to N and from O to P. The first year's growth being comprised within the 

 space E F P, the second within .that of O P M N, and the third within the 

 area M N D C. The pith occupies the space E F G. The rays of cellular tissue, 

 diverging from the centre to the bark, are seen in this tree to be 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 inter- 

 vals ; the second are the finer radial divisions, which in like manner are uni- 

 formly arranged and occupy the spaces between those of the first class. 



A peculiarity of structure belongs to the Oak and some other trees, which is 

 revealed in this section. By comparing this figure with that of the Pear, for 

 example, all the dividing lines of*the latter are seen proceeding from the centre, 

 but in this, numerous white waving lines are discerned running 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 6, &c., traverse the 

 wood in the same direction in irregular waving lines. The spiral vessels d c, &c. 

 are scattered in considerable numbers throughout the wood, occupying a large pro- 

 portion of its space. They vary much in size, the smallest, as d for instance, not 

 being more than one/owr hundredth of an inch is 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 



