STRUCTURE OP CONIFEROUS WOOD. 



cursory examination of the section shows the following arrangement of the 

 parts: 1st — A central pith, which is larger or smaller 

 according to the kind of tree or shrub to which the 

 branch belongs, large in the Elder, smaller in the 

 Sycamore, a mere point in the Oak; and also 

 according to the age of the stern; it is always 

 enveloped in a delicate network of vessels termed 

 the medullary sheath. 2nd — Around the pith will 

 be seen a series of rings or concentric circles, the 

 number of such rings corresponding precisely with 

 the age in years of the stem examined. 3rd — These 

 rings are crossed by lines generally of a lighter 

 colour, all radiating from the -central pith, and known 

 Pig. 1. Transverse and by the name of medullary ravs. 4th — The whole 



longitudinal sections of a- , , , -,-,-,. ., 



three years old stem. is surrounded by, and enclosed m, a cuticle or 

 covering familiarly called the bark. 



This form of structure is the result of the mode of growth of the 

 tree or shrub. The pith is the remains of the cellular or soft tissue 

 of the tender stem first produced from the seed in the primary shoot 

 and its prolongation during its successive annual growths, or from a 

 leaf bud in a lateral one. The concentric rings are the woody fibrous 

 tissues with the spiral and other vessels which lie next the centre, and 

 are the parts first formed by the growth of each succeeding year ; 

 the medullary rays are, as it were, the channels of communication 

 during the circulation of the sap; first, between the central pith and 

 the bark surrounding the first ring of woody fibre during its formation, 

 next between the first ring and the bark during the formation of the 

 second ring, and so on, the central portion becoming hardened, and 

 the channels obliterated by the gradual deposition of the inorganic 

 matter which is always contained in the fluid absorbed or sucked up 

 by the roots. As such a stem increases in size by successive layers 

 from without, the bark being as it were pressed outwards to admit 

 of the successive yearly addition of rings, it is technically called an 

 Exogen. 



The stem or branch of a Coniferous tree shows the same general 

 structure as that we have just described. The medullary rays are, 

 however, generally much fainter, and often quite inconspicuous, but they 

 are nevertheless present ; there are also differences and modifications in 

 the minute parts, which we will now point out. 



The woody fibrous tissues contained in the rings that surround the 

 central pith in the stems and branches of the broad-leaved and deciduous 

 trees, and in fact in nearly all Exogenous stems when first formed, are 

 usually minute elongated tubes joined together into a continuous threadi 

 Examined by means of the microscope, these minute fibres are found 

 to be elongated transparent cells, with membranous walls, tapering to a 



