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1 86 The Living Plant 



evident when preserves are made with plenty of sugar, but 

 fruits retain their shape in some of the processes where little or 

 no sugar is used. Dry raisins and currants become plump when 

 soaked, for their cells contain sugar though their protoplasm is 

 dead; and the process is hastened by the heat of cooking. The 

 crisping of celery or cucumbers when placed in water is a case of 

 increased turgescence, the tense cells actually exploding, as it 

 were, when crushed by the teeth. The reason, by the way, why 

 the water must be cold for best crisping is this, that warmer 

 water tends to drive out and replace the air of the intercellular 

 passages, thus deadening the explosive action in which crisping 

 consists. Moreover, the bursting of hard-skinned berries, like 

 cranberries, when heated in water, though apparently an os- 

 motic phenomenon, is primarily due to the swelling of the air 

 confined by the skin, the same thing which occurs in apples when 

 baking. A genuine osmotic bursting does, however, occur some- 

 times in fruits, like plums and grapes, while still on the plant, 

 because of a great absorption of water from the ducts by the 

 sugar-ripe cells under action of heat on bright summer days; 

 and the calyx of carnations sometimes bursts from the same 

 cause when the temperature rises in the greenhouse. There is 

 in tomatoes an osmotic disease, called (Edema, due to an over- 

 absorption of water by soft cells, and the consequent formation 

 of blistery swellings. The swelling of soaking seeds with a power 

 sufficiently great to result in the bursting of strong vessels, is 

 chiefly due to osmosis though it is partly imbibition, and the 

 same is true of the forcible swelling of dried apples. Sugar and 

 salt are common preservatives, the one of fruits and the other of 

 meats, though neither is really poisonous to the germs and molds 

 which cause decay, while the former is actually nutritive; but in 

 strong solutions they act germicidally, because they withdraw 

 so much water from the decay organisms as to render these 

 inactive. Moreover, either of these substances, when eaten in 

 more than moderate amount, causes thirst, which results from 



