However, a large number of examples may be cited where external morphology has proved inadequate and it is important to study the internal structures. Characters of the cuticle, Stomata, secondary xylem, glands, hairs, pollen grains, embryo sac, endosperm and seed coat have all proved to be of value.
To illustrate the significance of internal morphology and embryology a reference will be made, to the following taxa in particular : Ephedra, Exocarpus, Kingdonia, Paeonia, Phyllocladus, Scyphostegia, and Sequoia.
(1) Ephedra is more closely related to the cordaites and conifers than to the Gnetales and should be placed in a separate order Ephedrales ; (2) Exocarpus does not belong to the Taxaceae as proposed by one botanist or even close to it, but is a member of the Santalaceae ; (3) The genus Kingdonia should perhaps be removed from the Ranunculaceae and assigned to a new family Kingdoniaceae ; (4) Paeonia cannot be assigned to the Ranunculaceae and is not even related to Helleborus as suggested by Hutchinson ; (5) Phyllocladus is confirmed as a member of the Podocarpaceae all of whose genera are characterized by certain peculiarities in the development of the male gametophyte and the embryo ; (6) The genus Scyphostegia is in no way related to the Urticales, Celastrales or Monimiaceae ; its real affinities are still undecided ; (7) Sequoia gigantea is too different from S. sempervirens to be included in the same genus ; the former must be put in a separate genus Sequoiadendron as recommended by Buchholz.
Internal morphology generally corroborates the conclusions based on external morphology. However, it has a special value in cases of controversial nature. Sometimes it fails to provide an immediate solution but reorients our ideas in more fruitful directions.