knowledge of the A natomy of Plants . 1 49 
covery (1883) that the epidermis often acts as a water 
reservoir, may be mentioned. In the case of a many-layered 
epidermis, such as that in Piperaceae, his experiments are 
decisive, but it is very doubtful whether this view applies 
generally. As regards its origin the aqueous tissue does not 
of necessity belong to the epidermis at all, and thus we here 
have a good illustration of the difference between develop- 
mental and physiological classification of tissues. 
Then we have Mr. F. Darwin’s interesting explanation of 
the function of the waxy layer of the cuticle in preventing the 
wetting of the stomata, while as regards epidermal appendages 
Haberlandt’s very beautiful exposition of the structure and 
mode of action of stinging hairs may be noticed. Our knowledge 
of epidermal glands, and especially of extra-floral nectaries, 
has greatly increased, but this is a subject which has been 
dealt with elsewhere at this meeting. 
The dermal tissues of roots have been the subject of much 
interesting research. What we generally term epidermis is 
here an absorptive structure, but this is only the case in young 
roots. The older roots cease to be absorptive and require, 
like sub-aerial organs, a protective dermal structure. Olivier, 
especially, has shown how this protection is afforded by the 
outermost cortical layer, which becomes cuticularized and often 
exactly simulates a true epidermis, while the absorptive layer 
withers away. This epidermoidal layer, or better exodermis , 
is especially evident in monocotyledonous roots, which have 
a persistent cortex, and often no periderm, so that the exo- 
dermis here has to form a permanent protective structure. 
This does not, however, hold good of all Monocotyledons, for 
Olivier has shown that many of them develop a true external 
periderm on their older roots, whether aerial or terrestrial, 
e. g. Aroideae, Liliaceae, etc. 
The distinction formerly drawn in this respect between the 
roots of the two classes of angiosperms has been further 
broken down by Olivier’s discovery of an external periderm 
in many dicotyledonous roots (Ranunculaceae, Gentianeae, 
Primulaceae, etc.). 
