148 Scott . — On some recent progress in our 
starting-point. No attempt will of course be made to sum- 
marize all that has been done since that time. My object is 
only to call attention to some of the most striking advances 
in our knowledge, especially in the hope of showing how 
much room there is for further fruitful research in this field. 
Classification of tissues. This may rest on a purely ana- 
tomical, a developmental, or a physiological basis. In de 
Bary’s work the first principle is adopted, i. e. histological 
elements are classified in the main according to their mature 
structure, though the separate treatment of secondary tissues is 
so far a concession to the developmental method. The latter 
principle is impossible to carry out consistently, owing to the 
fact that in the embryonic tissues distinct histogenetic layers 
cannot always be distinguished. 
The physiological method of classification is characteristic 
of the present period of anatomy, and has been applied in a 
thoroughgoing way by Haberlandt. This tendency dates 
from Schwendener’s £ Mechanical Principle in the Structure of 
Monocotyledons,’ 1874; and culminates in Haberlandt’s 
‘ Physiological Anatomy of Plants/ 1884. The attempt to 
classify all tissues by their functions suffers under two dis- 
advantages. Firstly, it is premature, our knowledge of 
function being still so imperfect. Thus there is a danger of 
the classification being based on doubtful hypothesis, e. g. 
Haberlandt’s inclusion of laticiferous tubes under conducting 
tissues. Secondly, it has been carried out somewhat pedanti- 
cally, e. g. his treatment of the stomata as belonging to a 
different tissue-system from the epidermis. This might 
indeed have been avoided by defining dermal structures as 
those which regulate the communication between the plant 
and its environment. The work of the Schwendenerian 
school has, however, added immensely to the interest of 
anatomy, as we shall see in detail further on. 
Beginning with the dermal tissues as just defined, the sub- 
aerial epidermis has long been among the best known tissues, 
and the points of progress we have to note are rather 
physiological than strictly anatomical. Westermaier’s dis- 
