He was ably followed by Strasburger (Ueber den Bau und die Verrichtungen der Leitungsbahnen in den Pfianzen, Jena, 1891), Haberlandt and others.
The study of the specific functions of the various tissues) by Schwendener (,Das mechanische Princip un Ban der Monocotylen, 1874, and other works), followed by numerous pupils and others, among whom Haberlandt (Physiologische Pfianzen-Anatomie, Leipzig, 1st ed., 1884, 4th ed., 1909, and other works) is pre-eminent.
Recently some investigations by Haberlandt, Noll, Darwin and others have suggested an explanation which has much to recomrtiend it.
The response to the action of light in diatropic leaves is, according to Haberlandt, due to the presence of epidermal cells which are shaped like a lens, or with lens-shaped thickenings of the cuticle, through which convergence of the light rays takes place and causes a differential illumination of the lining layer of protoplasm on the basal walls of the epidermal cells, by which the stimulus resulting in the orientation of the leaf is brought about.
Notwithstanding the fact, however, that these cells are capable of acting as very efficient lenses the explanation given by Haberlandt has not been widely accepted and evidence both morphological and physiological has been brought forward against it.