IV.] RIPENING OP FRUITS 75 



etc.), until it has developed adequate roots and leaves of 

 its own. The broad chemical changes going on in the 

 plant, such as the migration of the starch from one part 

 to another, preceded by its conversion into sugar, and 

 even such changes as the desiccation and the conversion 

 of ^-proteins into proteins which marks the ripening of 

 cereals, are recognisable enough; there are, however, a 

 number of more subtle changes which we are unable in 

 the present state of knowledge to follow, though they may 

 often be appreciable in the bulk as constituting what 

 the practical man calls quality. As an example we 

 may instance the ripening of fruits such as apples and 

 pears, which after the migration and filling of the fruit 

 has been completed, will to a large extent go on 

 though the fruit has been detached from the tree. 

 We know the broad outline of the process : the fruit 

 becomes soft and sweet, often an intense harshness of 

 taste entirely disappears, in many cases fragrant essences 

 and aromas develop. Chemically we can recognise 

 that starch in the unripe fruit disappears because it is 

 converted in sugars, that some of the tough pectin 

 bodies are also converted into sugars, as also are the 

 very bitter tannins, while the agreeable flavoi'rs appear 

 to be due to ethers produced by the union of some of 

 the vegetable acids with alcohol derived from the partial 

 breakdown of sugar. In fact, if the respiration of fruits 

 is deranged by putting them in a jar containing no 

 oxygen, the sugars will not be converted into carbon 

 dioxide and water, but will be broken down part of the 

 way into the less oxidised form of alcohol, so that the 

 fruit may become alcoholic without any fermentation in 

 the ordinary sense of the word taking place. Fer- 

 mentation by yeast may be regarded as a similar 

 case of respiration in the absence of free oxygen ; the 

 yeast cells when placed in a liquid containing sugar 



