45 2 Peirce. — Studies of Irritability in Plants. 
always be avoided, and though there may be other spores introduced than 
those designed, such sterilization as this is efficient in destroying whatever 
was in the soil and the water at the start. 
As I wished to study the early stages in germination, I made a few 
cultures under somewhat different conditions. I placed a sheet of white 
filter paper over the moist soil before sterilizing, and afterwards enclosed 
the dishes in dull opaque black paper except for a slit on one side, one 
centimetre square, and reaching upwards from the top of the layer of soil. 
Thus whatever light entered these dishes entered from one direction only, 
with the rays mainly parallel. 
In another case I used Knop’s solution, sterilizing as before, placing 
an equal number of cultures on clinostats and on the shelves beside them. 
While in Naples in 1904 I began experimenting similarly with marine 
algae, and I have had no opportunity as yet to resume experimental work 
on sea-weeds, though I hope to do so presently. I will report such results 
as I obtained for whatever suggestive value they may have. 
Uniform sowing of spores on prepared surfaces — soil, paper, or water — 
is not easy, and as the young plants growing up close together shade each 
other, it is very important that the spores be sown sparingly, and be as 
uniformly distributed as possible. I have tried to dust them carefully 
from paper held at some distance above the dishes. By turning the dishes 
at the same time, some degree of uniformity in distribution is attainable, 
especially if only a small quantity of spores be used for each dish. 
By using dishes no deeper than the crystallizing dishes described, 
it is possible to place them on the stage of an ordinary microscope, and 
to examine them without in any way disturbing the cultures. It is 
necessary to make the camera drawings of clinostat cultures as rapidly 
as possible, to avoid too long exposure to light from one direction only. 
The exposure to the comparatively dry air of the laboratory for such short 
times, say five minutes, seems not to injure the plants, for I have taken 
pains not to keep the cultures very wet. Where it is necessary to make 
more careful or more detailed drawings, the only thing to do is to remove 
the little plant to be studied from the culture altogether, and this means its 
loss ; but I have made many rapid drawings in succeeding weeks of the 
same plants as they were growing, and in this way I have been able to 
follow the development of the same individuals. 
The cultures were all on shelves in windows facing south-west. Direct 
sunshine was avoided by window-shades of thin white holland, drawn up from 
below when needed. The temperature of the laboratory naturally ranged 
somewhat higher than that out of doors ; but it was, nevertheless, not so 
high by day as that of most American laboratories, artificially heated 
in winter, and at night it was not so low as out of doors. The result 
this year was a much more rapid and luxuriant growth in my cultures than 
