28 



WOOD AND FOREST. 



The cells of conifers are called tracheids, meaning "like trachece" 

 They are cells in which the end walls persist, that is, are not ab- 

 sorbed and broken down when they meet end to end. In other 

 words, conifers do not have continuous pores or vessels or "trachea?" 

 and hence are called "non-porous" woods. 



But in other woods, the ends' of 

 some cells which meet endwise are ab- 

 sorbed, thus forming a continuous series 

 of elements which constitute an open 

 tube. Such tubes are known as pores, 

 or vessels, or "tracheae," and sometimes 

 extend thru the whole stem. Besides 

 this marked difference between the por- 

 ous and non-porous woods, the porous 

 woods are also distinguished by the fact 

 that instead of being made up, like the 

 conifers of cells of practically only one 

 kind, namely tracheids, they are com- 

 posed of several varieties of cells. Be- 

 sides the tracheae and tracheids already 

 noted are such cells as "wood fiber," 

 "fibrous cells," and "parenchyma." Fig. 

 19. Wood fiber proper has much thick- 

 ened lignified walls and no pits, and its 

 main function is mechanical support. 

 Fibrous cells are like the wood fibers 

 except that they retain their proto- 

 plasm. Parenchyma is composed of 

 vertical groups of short cells, the end 

 ones of each group tapering to a point, 

 and each group originates from the 

 transverse division of one cambium cell. 

 They are commonly grouped around the 

 vessels (tracheae). Parenchyma constitutes the pith rays and other 

 similar fibers, retains its protoplasm, and becomes filled with starch 

 in autumn. 



The most common type of structure among the broad-leaved trees 

 contains tracheae, trachaeids, woody fiber, fibrous cells and paren- 

 chyma. Examples are poplars, birch, walnut, linden and locust. In 



Fig-. 19. Isolated Fibers and 

 Cells, a, four cells of wood 

 parenchyma; , two cells from 

 a pith ray; c, a single cell or 

 joint of a vessel, the open- 

 ings, x, x, leading into its up- 

 per and lower neig-hbors; rf, 

 tracheid; <, wood fiber proper. 

 After Roth. 



