463 
Wager. — The Perception of Light in Plants. 
of the epidermal cells should be first obtained ; then slowly raise the tube 
of the microscope and the images of the object focused through each of the 
epidermal cells will gradually come into view. If the images overlap reduce 
the size of the diaphragm opening until they are clear ; in most cases 
it will be found that an opening of from two to three millimetres is suitable. 
Now focus the image clearly by means of the special eye-piece supplied 
with the photo- micrographic apparatus ; replace it by the camera, in which 
a suitable plate has been placed, and expose. The correct exposure must 
be found by experience ; but using a fairly rapid plate with i-inch objective 
and ocular A (a moderately deep eye-piece) an exposure of from 2 to 
30 seconds is sufficient. With higher powers correspondingly longer 
exposures must be given. Thus with a bright image, using a §-inch 
objective and the apochromatic ocular No. 8 of Zeiss and Cadett extra- 
rapid plates, an exposure of one minute was given. 
The following method of mounting the epidermis of Trade scantia 
jiuminensis , or other epidermis which possesses fairly regular lens cells, 
is useful. Float the strip of epidermis on a 5 per cent, solution of chromic 
acid for a short time ; then transfer it to water. Cover a glass slide with 
glycerine jelly and allow to cool. Lay the epidermis on it and soak up the 
superfluous water. The cells will remain in good condition for observation 
for several hours. As the epidermis dries the outer walls of the epidermal 
cells sink in and form concave lenses through which images can still 
be focused, but below instead of above the layer of epidermal cells. 
By means of the autochrome plates of Messrs. Lumiere, simple colour 
photographs have been obtained through the epidermal cells of Trade scantia 
Jiuminensis and the special lens cells of Mesembryanthemum cor difolium. 
The objects used were bright-coloured flowers photographed by reflected 
light, and simple patterns constructed of coloured sheets of gelatine photo- 
graphed by transmitted light. In bright sunlight in July, using a i-inch 
objective and a No. 4 eye-piece, an exposure of from 20 to 30 minutes 
was given, but in a good light, out of the sun, an exposure of if to 2 hours 
was necessary. The 3J x 3J plates were cut into four if x if plates for this 
purpose. No difficulty was experienced in the development of these cut 
plates beyond a slight tendency to frill at the edges of the film where it had 
been broken across. 
A few illustrations produced from some of the photographs obtained 
are given in the plates which accompany this paper. Fig. 1 shows a repro- 
duction of a photograph taken through the epidermal cells of Tradescantia 
Jiuminensis , and Fig. 6, a photograph of a geometrical figure taken through 
the cells of Zebrina pendula. In both cases, the image was brought to 
a focus below the basal walls and the outlines of the cells cannot be seen. 
Fig. 12 shows images brought to a focus almost exactly upon the basal 
wall in Zebrina pendula through papillate projections in the middle of the 
