82 VIEWS OF THE MICROSCOPIC WORLD. 



oiilv a single ring' of vessels, or cluster of vessels arranged in a circle, is found 

 surrounding the wood, and this, with the tissue in which they are enveloped, con- 

 stitutes the bark of the plant. During- the second year, a new layer of bark with 

 its vessels and tissue grows within the former, and next to the wood ; and every 

 successive year a new layer of bark is thus added : the entire thickness of this 

 envelope being constituted of a series of layers, increasing in number from within. 



Next to the bark, the wood is found consisting likewise, as we have seen, of 

 vessels and cellular tissue, and the cross section of a plant or shoot of one 

 year's growth, exhibits the wood arranged around the pith, and composed 

 of a ring of vessels banded together by cellular tissue. The growth of the suc- 

 ceeding year gives rise to a new ring of vessels outside of the first ring, and be- 

 yond this second ring, a third circle of vessels appears during the third year. The 

 wood of the tree therefore increases from within outwards, in a direction contrary 

 to the growth of the bark ; and consists of a series of rings, equal in number to the 

 years indicating the age of the tree. The outer ring is whiter and more full of 

 sap than the rest, and has received from this circumstance the name of sap-ivood. 

 In the last annual layer of wood and bark, by which the trunk is increased in 

 size, the sap-wood and new bark are in contact ; but the layer of the next year 

 pushes up between, and separates them by its entire thickness. Every year a 

 new layer is thus interposed in the midst of the others, the last formed layers of 

 wood and bark touching each other, while the oldest are the most widely sepa- 

 rated ; the first ring of wood directly enclosing the pith and the first envelope 

 of bark constituting the outer surface. 



The layers of wood annually formed are not simple in their structure, but are 

 each composed of a great number of other layers. These delicate membranes 

 can be distinctly perceived in the oak by the aid of a common magnifying glass, 

 when the branch or shoot is cut obliquely. By macerating the rings of wood in 

 water, Du liamel was enabled to separate an annual layer into a vast number of 

 primary layers, which were thinner than the finest paper ; and he afterwards 

 discovered by experiment that these primary layers, constituting any annual ring, 

 were formed in succession, during the period of growth and vegetation in the 

 year to which the ring belonged. So that however curious it may seem, it is still 

 true, that not only does the relative thickness of each annual ring indicate the 

 comparative fruitfulness of every year of the existence of the tree, but each of 

 the primary layers composing the several rings tells, by its thickness, of the com- 

 parative vegetative energy of each week and day of the particular season to 

 which it belongs; and thus every tree becomes a record of the fertility of that 

 period of time during which it lived and flourished. The branch possesses 

 exactly the same structure as the trunk. 



