364 



Sir W. Thiselton-Dyer. On the Influence of 



municated the result to Professor Dewar, and he informed me, 

 August 15 : — " The temperature Fahrenheit to which the seeds were 

 cooled was - 453° F. below melting ice." 



These are the details of the experiment. As it is not likely to be 

 often repeated, I have thought it worth while to place them on record 

 as precisely as possible. 



The first question that suggests itself is, what evidence we have 

 for believing that the seeds have actually been brought to the almost 

 inconceivable temperature with which they were surrounded. That 

 they were so brought, Professor Dewar himself has not a shadow of 

 doubt. That substances at widely extreme temperatures can remain 

 in juxtaposition at least for some time, and still maintain them, is 

 illustrated by a striking experiment shown by Professor Dewar at the 

 Eoyal Institution on April 1, 1898. Liquid air poured into a large 

 silver basin heated to redness, remained apparently as quiescent at 

 this high temperature as in cooler vessels, and maintained a spheroidal 

 condition. This is well understood. But the fact remains that liquid 

 air with a temperature of about - 190° C. was contained in a vessel 

 which had a temperature of 800° C., the difference in temperature 

 between the two being 1000° C. 



If we turn for a moment to the effect of heat on living structure, 

 we know that a temperature of 75° C. is fatal to all protoplasm, 

 because at that temperature its proteids are coagulated. Yet there is 

 good evidence for the fact, that seeds have been exposed for pro- 

 longed periods to a temperature above 100° C, and yet have sub- 

 sequently germinated. It may be taken as absolutely certain, that in 

 this case that temperature never reached the embryo, but must have 

 been intercepted by the imperfect conducting power of the seed-coats. 

 Cohn again has found that the spores of Bacillus subtilis survive pro- 

 longed boiling,* and a similar observation applies. 



It is probable that plant structures are deficient in thermal trans- 

 parency, and they are notoriously indifferent conductors. Nevertheless 

 it is difficult to believe that in the case of such small bodies as seeds, 

 their being brought to the temperature with which they are surrounded 

 can be more than a question of time. 



That the thermal opacity of at least the seed-coats, may be really 

 considerable is not, however, impossible, even at low temperatures. 

 The following remarks by Professor Dewar -have an obvious bearing 

 on this point : — 



" Pictet, after an elaborate investigation, concluded that below a. 

 certain temperature, all substances had practically the same thermal 

 transparency, and that a non-conducting body became ineffective at 

 low temperatures in shielding a vessel from the influx of heat. Ex- 

 periments, however, prove, that such is not the case, the transference 

 * See Tine's < Physiology of Plants,' p. 283. 



