1 50 Scott. — On some recent progress in our 
In certain roots the periderm is specially modified to 
enable them to act as organs of respiration, as in the roots of 
Avicennia and Sonneratia described by Goebel. Similar 
structures have been observed by Jost in certain palms, etc., 
and the peculiar spongy periderm of the floating roots of 
Sesbania may also perform respiratory functions. 
Assimilating tissues. There is evidence that the palisade- 
tissue does on an average three to four times as much as- 
similating work as the spongy parenchyma. Stahl has shown 
that the palisade-cells are the form better adapted to high 
and the spongy cells to low intensities of light. Evidence of 
this is found in the phenomena of epistrophe and apostrophe, 
and in the variations in development of palisade-parenchyma 
according to the degree of exposure, as in the sun-leaves and 
shade-leaves of the Beech. Corresponding changes have been 
observed by Stahl in the leaves of £ Compass plants.’ 
Pick’s experiments showed that the length of palisade-cells 
varies with the intensity of light, and that the inclination of 
their long axes depends on the direction of the incident rays. 
Plis experiments also demonstrate a different organisation of 
the chlorophyll-containing tissues on the north and south 
sides of assimilating stems. 
Haberlandt’s view that the form and arrangement of palisade- 
cells are adaptions for the direct conduction of assimilated 
food appears to be only of limited application. 
The absence of differentiation of the mesophyll in chlorotic, 
and therefore non-assimilating leaves is worth noticing here. 
Mechanical tissues. Schwendener’s exposition of the prin- 
ciples on which these tissues are arranged, especially in 
Monocotyledons, may be assumed to be familiar. It is 
remarkable that these elaborate adaptations are confined to 
organs destitute of secondary thickening. In the stems of 
dicotyledonous trees, especially those with heart-wood, the 
most effective mechanical tissues usually lie towards the 
middle of the stem. There is a tendency among physio- 
logical anatomists to make mechanical adaptation explain too 
much. Haberlandt’s explanation of the medullary zone of 
