68 OUR WOODLAND TREES. 
roots swelling as the Tree developes, exercise so 
much force that they have often been known to 
destroy buildings by displacing the stonework 
of their basements. 
Meanwhile, during this process of root develop- 
ment the marvellous power of growth has been 
manifested above ground. The plumule has been 
elongating from its root pedestal into the stem, 
which is the mainstay of the plant—the column 
which is to support its system of branches, leaves, 
flowers, and fruit. The formation of the stem in 
flowering plants gives occasion for their division 
throughout the vegetable kingdom into two great 
classes, called respectively, Hwogens and Endo- 
gens, terms suggested by the facts that whilst 
the first-named class comprises plants whose 
growth is effected by additions to the stem or 
trunk outwards, Hndogens increase by internal 
growth. The tissues of Hxogens, too, are ar- 
ranged upon a different plan from those of 
indogens. The differing modes of growth and 
of the formation of the stem chiefly distinguish- 
ing Exogens from Hndogens, we need not give 
in elaborate detail. All our forest Trees being 
