4 Scientific Proceedings, Royal Dublin Society. 
So that we may say that the total mechanical effect produced in ~ 
dry seeds by immersing them in liquid hydrogen is very slight. 
In the case of moist seeds there is an entirely different series 
of events. Here the wet seed-coat is an efficient conductor of 
heat, and the cells of the seed contain a much larger percentage 
of water. Consequently, freezing of the tissues is only a question 
of temperature. At what temperature freezing of the above- 
mentioned seeds begins, I have not attempted to determine. 
Probably it varies with the species, and with the actual amount 
of water contained in the seed. ‘That a plant-tissue may he 
frozen without being killed, seems fairly well established by 
experiment ; and probably this holds good in the case of seeds 
also. That on still further lowering the temperature, a point is 
reached at which freezing to death occurs is equally certain from 
the foregoing experiments. The older theory of freezing was 
that a formation of ice particles took place within the cell. 
The more modern theory is that as the temperature falls, con- 
traction of the protoplasm takes place, water passes out from 
the cell through the cell wall, and freezes outside the cell in the 
intercellular spaces. The cell-sap left behind in the cell will 
therefore be more concentrated, and will require a still lower 
temperature to freeze it. Ifthe temperature falls further, more 
and more water will pass outside the cell and become frozen. On 
this theory, if freezing has not gone too far, and if the thawing 
be gradual, the ice in the intercellular spaces melts, and is 
absorbed by the cells again, no loss of vitality resulting. 
Freezing to death, however, is more difficult to account for. 
Ewart! holds that protoplasm, in order to retain its vitality, 
requires at least 2-3 per cent. of water. Matruchot and Molliard’ 
believe that, in freezing, exosmosis of water goes on until the 
protoplasm of the cell, and especially of the nucleus, contains less 
than the minimum required to maintain vitality. Molisch believes 
that death by freezing is the same thing as death by desiccation. 
Tf these theories are correct, and we assume that, as continual 
cooling of the cell goes on, more and more water is withdrawn 
until the protoplasm contains less than 2-3 per cent., the same 
argument should apply to the dry seeds which contain about 
1 Trans. Liverpool Biol. Soc., xi., 1897. * Comptes Rendus, cxxxii., 1901. 
