ANATOMY OF WOOD, ]1 



The parencbymatous cells are usually brick-shaped ; they 

 are more rarely fibres with simple thinnings, termed pits, in 

 their walls. These pits are small whenever parenchymatous 

 cells adjoin one another, but large when they adjoin air-con- 

 ducting vessels. The parenchymatous cells retain their 

 contents in conifers, until the sapwood becomes converted 

 into heartwood ; in broadleaved wood they do so still longer : 

 they are concerned with the storage and chemical conversion 

 of nutritive material, and probably also with water-transport. 

 According to the order of their arrangement, there are bands 

 of parenchyma extending radially, termed medullary rays ; 

 also longitudinal parenchyma extending vertically alongside 

 the vessels, but sometimes scattered in the wood, especially 

 in the last layer of the summer-wood, and enclosing resin- 

 ducts. 



Of the elongated, spindle-shaped cells there are three forms, 

 tracheids, bordered-pitted cells, and libriform or scleren- 

 chymatous fibres. The first two form the wood of conifers, 

 but also occur in bmidleaved wood; (luring their first yi-ar, 

 after their walls have thickened, these elements lose their 

 contents, conduct only air and water, and are occupied chiefly 

 with water-transport. Libriform or sclerenchymatous fibres 

 are cells with highly thickened walls and very small simple 

 pits; they lose their contents in their first year and conduct 

 water and air only, being employed for water-transport ; they 

 do not occur in conifers. 



Fibrous cells are spindle-shaped parenchymatous cells with 

 similar functions to those of sclerenchyma. 



It is easy to see that all the above organs, especially those 

 with thick walls, tend to strengthen the wood, but the lateral 

 union of the cell-forms in the woody tissue is no less 

 important than the structure of the cells for the strength of 

 wood. 



The construction of the bole of a tree is explained best by 

 observing the formation of an annual shoot, which is similar 

 to that of a herbaceous plant. This in trees is a prolongation 

 of the previous year's shoot and is connected with all the 

 lower parts of the tree, down to the root-tips. 



If from a bud, c.y., of a spruce, the scales are removed, a 



