Stem Thickening

 
 
As woody dicot stems age, they get thicker.  But how does that happen?  Think back to the last page (vascular bundles).  What layer of cells is found in the vascular bundles of dicots that is not found in monocots?  If you recall, we said it is a single layer of cells located between the xylem and the phloem.  The picture to the right identifies it as the cambial zone, but we will refer to it as the vascular cambium.  The vascular cambium, as we discussed before, is a lateral meristem as is the cork cambium. 

What happens at a meristem?  A meristem is a region of rapidly dividing cells, of which there are two general classes.  Apical meristems are located at the tips of the stems and roots.  Here, cells rapidly divide in order to increase the length of the roots and shoots.  The lateral meristems are responsible for secondary growth, which makes the stems thicker. 
 
The vascular cambium separates the phloem and xylem.

 
How does it work?

Now we know that lateral meristems are responsible for the thickening of woody dicots, and we know that this region is called the vascular cambium in the stem.  But what actually happens in the vascular cambium and the cork cambium, and how do some trees get so thick? 

It all starts with the division of cells.  Select this link and read to learn more.

 
Ahh, Thicker Plants are Older Plants
(Dendrochronology)

Well, not really.  But a plant with more rings is older.  Most people have some idea that the age of a tree can be reliably found by analyzing the number of rings in a cross section of the stem.  When one uses the information from rings to interpret passed events, it is called dendorchronology.  Dendrochronology involves analyzing the rings of stems to determine what significant events occured during the life of the plant.  Events such as fires, droughts, and climatic changes can be identified. 

View this little movie to learn more about dendrochronology.  Be sure to take notes!

 
It is important to note that not all plants that have wood-like stems follow this same process.  A palm tree seems to be woody, but it is a monocot, and monocots do not have any vascular cambium. Thus, it is not a true woody plant. 

Remember, wood is secondary xylem.  The secondary phloem is sloughed off as the plant grows. 
Heartwood is older material that is no longer living and serves little in terms of water conductivity.
Sapwood is newer xylem that is lighter in color and actively transports water.

To finish monocot and dicot stems, we will examine the external features of a dicot stem section.