STEMS AND BRANCHES AND THEIR USES 207 



thousands of pounds. There are strains due also to the pres- 

 sure of the wind, to the occasional brushing of men and 

 animals against the plant, to the fall of other plants against 

 it, and the like. The strains brought to bear upon a stem are 

 largely sidewise strains, and it must be able to give way more 

 or less before a specially severe strain by bending without 

 breaking, and must be able to straighten itself after the strain 

 has passed. 



Seed plants have met the need for this particular kind of 

 strength in their stems by developing thick-walled cells that 

 are longest in a vertical direction. The greater part of the 

 trunk of a tree is composed of wood, and the wood, as we have 

 seen in the pine, is made up largely of long, thick-walled 

 cells, which help in strengthening the trunk as well as in con- 

 ducting the sap. There are other long, thick-walled cells 

 in the wood of the oak and of many other angiosperms (but 

 not of gymnosperms) , called wood fibers ; as a rule these are 

 narrower and shorter than the conducting cells. The fibers 

 are strengthening cells but do not conduct sap. It is plain 

 that if the various long cells in the wood ran crosswise instead 

 of lengthwise of the trunk, the trunk would not be able to 

 withstand the pressure of the wind but would be broken across 

 by a relatively slight strain. 



On the other hand, it is because these cells run lengthwise 

 that the wood of a tree is easily split in that direction. We 

 say that a split " follows the grain of the wood " — that is, 

 it runs between the long wood cells, and if their course is for 

 any reason twisted or wavy, the split surface is not smooth . A 

 " knot " is a place in the stem from which a branch grew. At 

 that point many of the wood cells of the stem ran diagonally 

 or crosswise to connect with those of the branch ; this cross- 

 wise course of many of the cells makes it difficult to split 

 through a knot. Strengthening fibers similar to wood fibers 

 occur also in the bast. In some plants these bast fibers are 

 very long, and they are important elements in the strength 



