at Different Degrees of Constant Temperature. 243 
sand—a proof that uniformity of temperature was preserved in 
these two positions, in consequence of the favourable arrange- 
ment of the experiments. 
The determination of the moment at which germination takes 
place is a delicate point, and to a certain extent arbitrary. The 
embryo alters within the seed before it appears outside, the 
radicle elongates more or less quickly, and, according to the 
species, the young plant shows itself in various ways. I have re- 
garded as the moment of germination, that at which, the sper- 
moderm being ruptured, the radicle begins to escape*. 
Several thermometers were at my command, most of them 
being graduated upon the tube itself. Although they were 
carefully made, I verified in each case the correction requisite 
to be made at zero ; and for the higher degrees, [ compared them 
at every ten degrees witha very accurate standard thermometer, 
belonging to the Geneva Society for the construction of philo- 
sophical instruments. This thermometer had been verified by 
M. Louis Soret, by passing a drop of mercury from place to place 
in the inner column. The principal cause of error arises from 
the difficulty of determining the fractions of degrees in thermo- 
meters constructed of thickish glass, when placed in various po- 
sitions, and when the eye is not always perpendicular to the tube. 
I hope, however, that the numbers are accurate to within one- 
tenth of a degreet. 
The object of my experiments being to observe the germina- 
tion at different but constant temperatures, I had an apparatus 
constructed under the direction of Professor Thury, which was 
satisfactory as regards the temperatures near 0°, but which was 
not found sufficient for the other circumstances. This apparatus 
consists of a cubical zine reservoir, 44 centimetres in each di- 
mension, surrounded by sawdust and contained within a large 
wooden box. The reservoir could be filled with ice or water of 
a given temperature, and the vessels or bottles containmg the 
seeds could be arranged either in the reservoir or in the sawdust, 
or even in the compartments of a zinc projection springing 
from one of the faces of the box. This lateral addition did not 
answer, because its cavities did not afford fixed conditions of tem- 
perature and it occasioned a loss of part of the advantages, in 
* M. Burckhardt, in the experiments alluded to above, and with which I 
was not then acquainted, regarded the moment at which the cotyledons 
became exposed as the period of germination. This is rather a period of 
vegetation. It may be of value in the comparison of the same species 
under different conditions, but it varies greatly in the case of different 
species, certain embryos remaining for a long time recurved under the 
surface of the soil or with their cotyledons imprisoned in the remains of 
the spermoderm. 
t The degrees are always those of the Centigrade thermometer. 
16# 
