MINUTE STRUCTURE OF CONIFEROUS WOOD. 



83 



Fig. 43. Transverse section 

 of two wood-cells (tracheides) 

 of Scots Pine, each with a 

 pore </; hi, intercellular spaces. 

 x 400. Copied from Thome. 



constitute heart-wood fibre. The tracheides formed in spring have 

 thinner walls and larger cavities than those formed in summer which 

 have thicker walls and are more compressed. This is the cause of the 

 difference observable in the inner and outer 

 zone of the annual rings mentioned above. 



" Bordered pits," although not peculiar to, nor 

 found in all coniferous wood, are a conspicuous 

 anatomical feature in the most important 

 coniferous woods used for economic purposes. 

 Their form and position are best seen in a 

 rapidly growing herbaceous shoot of the Scots 

 Pine or any of the common Pines. "When 

 the cell-wall begins to thicken, comparatively 

 large spaces remain thin, but as- the thickening 

 augments, it reaches even the thin spaces. 

 The outline of the thin spaces of the wall in 

 the wood of Pinus sylvestris (also of P. Laricio 



and P. excelsa, see Figs. 45 47) appear circular on a front view ; the 

 edge of the thickening mass which arches over it grows also in a 

 circular manner and contracts the opening ; thus the front view of such 

 a pit shows two concentric circles, the larger of which corresponds to 



the original thin space, and 

 the smaller to the inner edge 

 of the thickening. ^s"ow since 

 this process takes place on both 

 sides of a partition wall of two 

 cells, a lenticular space is 

 enclosed by the two over-archiiigs 

 divided in the middle by the 

 original thin cell-wall, each 

 half of this pit cavity communi- 

 cating with the cell cavity by 

 a circular opening. When the 

 wood-cells lose their protoplasm 

 and become filled with air and 

 water, the thin cell-wall dis- 

 appears and the two pits form 

 a single cavity which is bounded 

 by the over-arching thickening 

 mass, and is united with the 

 adjoining cell cavities by a 

 circular opening."* 



The annual rings are, traversed 

 radially from the pith to the 

 bark by medullary rays, the 

 earliest formed only reaching 



Fig. 44. 1, Transverse section of part of one year's into the first-formed rinox thp 



growth of stem of Piniis excelsa. M, medullary . . gS ' m6 



rays ; K, resin-ducts, x 150. others originating in later ones. 



These rays in respect of their 



number and size have some influence on the technical properties of the 

 \vood.f The medullary rays are differentiated from the other tissues 



" Sach's Text Book of Botany," Vines' Translation, page 23. 

 t Schlich Manual of Forestry, Vol. V. p. 8. 



