THE WOOD FIBERS 79 



meant the measure of the force required to stretch the fibers 

 to the point beyond which they are unable to return to their 

 original length.) In this respect they are quite equal to wrought 

 iron, and in some instances they equal and even surpass steel. 

 But they surpass both iron and steel in a respect of great impor- 

 tance to plants: they are able to stretch many times as much as 

 the former before the limit of elasticity is reached. This quality 

 enables plants to bend before the wind and spring back to their 

 original positions when the stress is past. 



When the bast fibers have reached their full development 

 they die and their cell cavities become filled with water or air, 

 but they continue none the less effective as skeletal tissues. 



The lengths and frequency of occurrence of the bast fibers 

 often afford good evidence in the detection of the purity of 

 powdered drugs. 



The Wood Fibers. — The wood fibers are those bast-like 

 fibers that occur in the wood or xylem portions of stems and 

 roots. In most cases they are the product of the cambium 

 alone, and not of the procambium, and we therefore expect to 

 find them in the secondary, but not in the primary xylem. While 

 they are similar to the bast fibers in being elongated and tapering 

 and having thickened and lignified walls, they do not equal 

 the bast in length. They are rarely as long as 1.5 mm. and are 

 known to be as short as 0.3 mm. Since, as a rule, they are 

 derived from the cambium only, they do not occur as close to 

 the growing apex as do the bast fibers, but farther back where 

 secondary increase in thickness has begun (Fig. 11). 



The proportion of wood fibers to the other elements of the 

 secondary xylem varies greatly in diflferent species. In many 

 woody Dicotyledons the wood fibers constitute the greater part 

 of the secondary xylem or wood; but even in this class of plants 

 cases occur where the other elements of the xylem greatly 

 preponderate. 



The specific gravity, hardness, elasticity, and strength of the 

 wood depend upon many factors, not all of which are referable 

 to the wood fibers themselves. The proportion of wood fibers 



