HYGROSCOPICITY 177 



remain in the room for a few days, it replaces the water lost in the oven 

 by abstracting it from the air and returns to its original air-dried weight 

 of 50 grammes, subject only to the ordinary hygroscopic variation. 

 Precisely the same air-dried weight is ultimately reached if the fresh 

 material is put at once in the oven. In the same way, the leaf that 

 dries and dies naturally on the plant, if placed in the air after the heat 

 test, regains the water lost in the oven. This is Berthelot's principle 

 of reversibility (p. 149). 



(5) The author, after confirming this principle by appealing to his 

 experiments on Hazel leaves, shows that it presents us with the simplest 

 mode of differentiating the water-contents of plant-tissues, namely, 

 into (a) the water of hygroscopicity, which they hold whether living or 

 dead, and (b] the water of vitality, which they lose when they die 

 (p. 151). 



(6) He points out that the chemist, when producing by synthesis 

 organic vegetable substances, would allow the atmosphere to supply the 

 water of hygroscopicity, whilst he himself in his creative role would 

 have to furnish the water of vitality. To emphasise this point he 

 gives the result of an experiment on bread, and shows that just as 

 moist fresh plants after being exposed to a temperature of 100 C. can 

 only recover from the air the water lost by air-dried plants in the oven, 

 so bread after the same heat test only gains back what was originally 

 in the miller's flour, but cannot recover the water the baker put into it 



(P- r 53)- 



(7) After giving a further illustration of the principle of Berthelot 



in the results of an experiment on the seeds of Phaseolus multiflorus^ the 

 author proceeds to deal with the implications of this principle (p. 154)- 



(8) The first is that air-dried resting seeds, both permeable and 

 impermeable, contain only the water of hygroscopicity ; and the 

 second, which follows from it, is that there is in such seeds no water 

 associated with any vital function (p. 155). 



(9) Since some seeds can retain their germinative powers after 

 being desiccated to an extreme degree, the presumption arises that 

 in their resting state they can dispense with water altogether ; and 

 it is urged that we are not called upon to assign a function to the 

 minute amount of water that may remain after extreme desiccation. 

 A perfectly dry seed protected from the air alone possesses the 

 potentialities of immortality (p. 156). 



(10) Instead of favouring the longevity of a seed, water is 

 regarded here as the source of its greatest danger. Appeal is then 

 made to the biological significance of hygroscopicity in seeds, and 

 it is shown that whilst the constant hygroscopic reaction limits the 

 life of permeable seeds, its absence in impermeable seeds ensures 

 their longevity. The seed that has the best chance of living for 



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