272 THE STEUCIUEE OF TIMBEK. 



The comparative scarcity of protoplasm in these elongated 

 cells is the natural consequence of the volume of the cell in- 

 creasing without any corresponding increase taking place in the 

 protoplasmic contents. Instead of these filling up the cell they 

 now form a thin pellicle on the inside of the wall, like the plas- 

 ter in a room, with, in most cases, a few threads running across 

 from one wall to the other. 



It is in this part of the shoot that most of the height-growth 

 of the tree takes place, and it is there that the cells assume the 

 shape which they are permanently to retain. But so far the cells 

 are limp and pliable if the osmotic pressure, or turgidity as it is 

 called, is relieved, as for instance it is by cutting oS a shoot and 

 allowing the water to escape by evaporation. To ascertain how 

 permanent rigidity in woody cells is secured, we must examine 

 the shoot a little further back than the region where cell-elonga- 

 tion is chiefly taking place. This portion we may call the zone 

 of internal development of the cells. Of course there is no hard 

 and fast line between the portion of the shoot where the cells 

 are elongating under the influence of osmosis and the portion 

 where such elongation has practically ceased, but it is convenient 

 to look at the subject as though there were. The cells, then, 

 having attained to their full size, begin to pass over into the 

 condition of permanent and durable tissue. This condition is 

 induced by the protoplasm, which sets about thickening and im- 

 pregnating the elastic cellulose cell-wall with lignine or woody 

 substance. When this process has fairly begun the cells lose 

 their elasticity, for lignine being inelastic prevents any further 

 stretching. The whole of the cell-walls are not covered or im- 

 pregnated with lignine, for thin spots are left at numerous 

 places to permit of osmotic communication between adjoining 

 cells, but with the exception of these spots, which ultimately 

 appear as depressions and are frequently known as "pits," the 

 cell-walls become hard and rigid. When all the protoplasm has 

 disappeared the cells are dead, and it is cells in this state that 

 form the great bulk of all timbers. 



Summing up what has been said, it amounts to this, that the 

 height-growth of a tree is entirely coniined to the youngest 



