RIPENING OF FRUITS. 305 



cess of this gum-like substance is secreted and forms a firm 

 coating upon the exterior of the fruit. 



6. The sugar occurring in ripe fruits is evidently derived from 

 various sources. The occurrence of a large quantity of starch in 

 many unripe fruits, especially in apples, and its complete disap- 

 pearance at the time of ripeness, allow of no other explanation 

 than that the sugar occurring in fruits is formed by the conversion 

 of the starch under the influence of the acids present ; other in- 

 different substances, such as gum, vegetable mucus, etc., undergo 

 similar transformations and yield in this manner a certain portion 

 of sugar. Even tannin, which occurs in all unripe, but not in 

 ripe fruits, can be changed by acids and ferments so as to form 

 sugar. 



Thus far nothing justifies the supposition that the acids in 

 fruits, such as tartaric, citric, malic acids, are converted into 

 fruit-sugar. To entertain such an opinion it would have to be 

 supposed that the molecules of these acids, which are far more 

 simple than those of fruit-sugar, become more complex and are 

 converted into sugar ; in such natural transformations the reverse 

 is, however, generally the case, the molecules always endeavoring 

 to become the more simple the farther they withdraw from organ- 

 ized structures. 



7. It has been attempted to explain in various ways the very 

 remarkable phenomenon of the gradual disappearance of the 

 acids in ripening fruits. It might not be impossible that the acid 

 of a fruit is neutralized by the bases conducted to it through the 

 juice ; or that it is covered by the sugar or the mucous substances 

 formed in the juice ; or, finally, that it disappears at the moment 

 of ripeness by suffering actual combustion. An examination of 

 these various opinions leads to the conclusion that the acid is 

 neither neutralized nor covered by the sugar or the mucous sub- 

 stances, but that it actually undergoes slow combustion. 



During development and ripening a fruit passes through two 

 different stages sharply separated from each other by definite 

 chemical phenomena. In the first stage, which may be desig- 

 nated as that of growth, whilst the fruit remains green, its rela- 

 tion to the atmosphere appears the same as that of leaves, for it 

 absorbs carbonic acid and evolves oxygen. During this epoch it 

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