140 FORMATION OF THE SEED, AND RIPENING OF THE FRUIT. 



ever, the plant has in general, and especially if an annual plant, reached 

 nearly to maturity, and woody fibre is little required. The most im- 

 portant of its remaining functions is the production of the starch and glu- 

 ten of the seed, and of the substances which form the husk by which the 

 seed is enveloped. 



In the first stages of the plant's growth, the starch of the seed is 

 transformed into gum and sugar, and subsequently, when the leaves are 

 expanded, into woody fibre. In the last stages of its existence, when it 

 is producing the seed, the sugar of the sweet and milky sap is gradually 

 transformed into starch — that is to say, a process exactly the converse 

 of the former takes place. 



We are able, in some measure, to explain the mode and agency by 

 which the former transformation is effected — the latter, however, is still 

 inexplicable. We can ourselves, by the agency of diastase, transform 

 starch into sugar ; and, therefore, can readily believe such transforma- 

 tions to be effected in the young plant ; — but we as yet luiow no method 

 of re-converting sugar into starch ; and, therefore, we can only hazard 

 conjectures as to the way in which this change is brought about in the 

 interior of the plant during the formation of the seed. 



It is said that nitrogen is given off* by the flower leaf. We know that 

 this element is present in the colouring matter of the petal, and that it is a 

 necessary constituent of the albumen and gluten, which are always as- 

 sociated with the starch of the seed. It is plain, then, that the nitrogene- 

 ous substances [substances containing nitrogen,] contained in the sap at 

 all periods of the plant's growth, are carried up in great quantity to the 

 flower and seed vessel. These substances are supposed to be concerned as 

 immediate agents in effecting the transformations which there take place. 

 More than this, however, we cannot as yet venture even to conjecture. 



II. RIPENING OF THE FRUIT. 



In these plants, again, which invest their seed with a pulpy fruit — in 

 the grape, the lemon, the apple, the plum, &c. — other changes take 

 place, at this period, of a more intelligible kind, and other substances are 

 formed, on the production of which less obscurity rests. At one stage of 

 their growth, these fruits, as has been already stated, are tasteless — in the 

 next, they are sour — in the third, they are more or less entirely sweet. 



I. In the tasteless state they consist of little more than the substance 

 of the leaf— of vascular, or woody fibre, filled with a tasteless sap, and 

 tinged with the colouring matter of the green parts of the plant. For a 

 time, this young fruit appears to perform in reference to the atmosphere 

 the usual functions of the leaf— it absorbs carbonic acid and gives off oxy- 

 gen, and thus extracts from the air a portion of the food by which its growth 

 is promoted, and its size gradually increased. 



II. But after a time this fruit becomes sour to the taste, and its 

 acidity gradually increases — while at the same time it is observed to 

 give off a less comparative bulk of oxygen than before. Let us consi- 

 der shortly the theory of the production of the more abundant vegetable 

 acids contained in fruits. 



1°. The tartaric acid which occurs in the grape is represented by 

 C4 H, O5 (p. 124). 



There are two ways in which we may suppose this acid to be formed 



