96 THE NATURE AND WORK OF PLANTS 



125. Structure of stems. — If a small branch of 

 the elm, beech, maple, or chestnut is cut off and then 

 a portion of the branch is split lengthwise, it may be 

 seen that a hard covering of bark encloses the whole. 

 Immediately inside this is a layer of soft tissue of 

 material easily crushed and full of water. This 

 material is living while the bark was chiefly made up 

 of dead tissues. Small extensions of the living tissue 

 penetrate the wood which occupies the greater part 

 of the volume of the branch. In the centre is to be 

 seen a small amount of soft pith, which may easily be 

 cut or torn, and is generally made up of dead cells in 

 older branches. The branch is thus seen to be com- 

 posed of living and dead cells, and the dead tissue 

 greatly exceeds the living in bulk. 



126. Uses of stems. — The purpose of the stem 

 is twofold : to hold up the leaves and flowers, and 

 to conduct water and food between the leaves and 

 roots. The method by which these functions are 

 carried out may be best understood after a study 

 of the scheme in which the tissues are arranged iu 

 the stem. This has been seen roughly in the branch 

 of the tree examined, but it will be necessary to see 

 the manner in which the different parts of the living 



