Undoubtedly there's much information on this topic.
Errera's Law says cells divide along the shortest plane, however trees defy that law.
The attached PDF, remarkable for it's age, goes into some detail.
Also there's this PDF which shows the growth taking place.
http://www.treeworld.info/attachment...ll4-callus.pdf
Whilst it's confirmed there are cells of varying sizes when the cells do divide the grow to the size of the parent cell. There's also some information showing the size/age of tree determines cell size, a gradual change as the tree gets bigger and older.