84 



RESIN- DUCTS. 



Fig. 45. 2, Radial section of a one year 

 old stem of JHnns Lancio showing "pits" of 

 wood-cells and part of a medullary ray. 4, A 

 pit of a wood-cell in transverse oblique 

 section, x 150. 



in the cambium; the cells are less elongated than the tracheides and 

 are composed of somewhat different elements, being mostly with 



cellulose walls and arranged some- 

 thing like bricks in a wall trans- 

 versely to the wood-cells. 



Interspersed among the tracheides, 

 and most conspicuous in the summer 

 wood of the outer annual ring are 

 intercellular spaces running parallel 

 with the stem, called resin-ducts 

 which are a distinguishing feature 

 of coniferous wood ; they are formed 

 of a cylinder of large irregular cells 

 with very thin Avails. In these 

 ducts turpentine is secreted, which 

 is not only of itself an important 

 economic product, but also has 

 important effects on the technical 

 properties of the wood. Resin-ducts 

 also occur in the medullary rays so 



that " the whole of the wood may become impregnated with 

 turpentine which on exposure to the air may become oxydised into 

 resin. The quantity of resin in coniferous wood 

 varies with its specific gravity, and depends on 

 the greater or less development of the summer wood 

 in or near which most of the ducts are formed."* 

 Fig. 44 shows the position and form of two resin- 

 ducts in a transverse section of an herbaceous 

 shoot of Pinus excelsa at the end of the year's- 

 growth, and Fig. 48 shows a resin-duct "in a 

 transverse section of part of an herbaceous shoot 

 of Pinus Laricio. 



The bark of coniferous trees which has for the 

 most part but a temporary duration, is composed 

 of cork tissue which, when first formed on the 

 herbaceous shoot, is made up of cells similar in 

 structure to those of the pith. To keep pace with 

 the growth in thickness of the enclosed wood 

 cylinder, the cells, of which the young cork 

 tissue is made up, undergo relatively rapid 

 changes during the growing season ; they are 

 formed by bi-partitioii from the adjacent cambium, 

 and their walls soon become suberous. The 

 innermost layer of cork tissue thus formed is 

 termed phellogea, and from this new layers arc 



Fig. 46. TangeTtiai section of constantly formed in a centrifugal direction.! The 

 wood of Pi-mis excelsa showing layers of cells next the epiderm soon lose their 



the medullary rays and pits m the %, , A , \ , , , 



walls of the wood-cells, x 150. cell-contents and as the bark already lorined is 



* Sclilich, Manual of Forestry, Vol. V. p. 14. 



t According to Sanio, it also happens that when cork is beginning to be produced,, 

 the formation of permanent cells proceeds centripetally, or an alternation of centripetal 

 and centrifugal cell-formation takes place in the young cork tissue. "Sach's Lehrbiich,/" 

 Vine's Translation, p. 107. 



