Problems turning working programs into executable Jars - jar

Question

I'm running into problems when I turn my (working) java into an executable jar file.
I have a very simple number sorting program written. It's spread across two files (TreeGui.java and SortTree.java) that each do more or less what they say. TreeGui has an input textarea, a button and an output area. Pressing the button passes the input text to SortTree, which returns nicely sorted numbers that appear in the output box.
When I compile each file, and then run TreeGui, everything works fine
javac TreeGui.java
javac SortTree.java
java TreeGui
...runs everything lovely.
Now, when I try and put those classes into a jar file I keep bumping into the same string of problems. I have a manifest (TreeGuiManifest.txt) with the following (and a blank new line):
Main-Class: TreeGui
but when I put the whole lot together:
jar cmf TreeGuiManifest.txt TreeGui.jar TreeGui.class SortTree.class
I hit problems. Whether I run it directly by selecting the jar file or by using "java -jar TreeGui.jar" in the console, the program will load fine but the moment the button is pressed it throws a whole string that start like this and carry on for a long way:
Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: SortTree TreeNode
at SortTree.treeInsert(SortTree.java:35)
at SortTree.main(SortTree.java:24)
at TreeGui.actionPerformed(TreeGui.java:66)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
etc. etc.
It's clearly struggling to find elements in the SortTree code that pressing the button in TreeGui demands but I'm not sure what I can do to make it work.
Am I missing something really obvious? I'm using a variety of imported java.utils.*; etc, do I have to mention them when I make my jar file? Thanks for any help.
(The only reason I haven't pasted the actual code of the programmes here is that it all works perfectly when it's not a Jar file... I'll happily add them if necessary though)