782 PHYSIOLOGY. 



The changes prodaced upon the atmosphere in the maturation 

 or ripening of the fruit, depend upon the nature of the pericarp. 

 Thus, when the pericarp preserves its green state, as also always 

 when first formed, it has an action similar to that of the leaves ; 

 but when of other colours than green, and more especially when 

 succulent, it evolves carbonic acid gas at all times, instead of 

 oxygen, under the influence of solar light. 



Chemical Constitution of Fruits. — The chemical constitution 

 of fruits varies according to their nature and age. When 

 the pericarp is of a dry nature, it commonly assumes a whitish 

 or brownish colour, and its cells become incrusted with har- 

 dened matters {lignin). Under such circumstances, no further 

 •changes take place in its chemical constitution, and its vital 

 activity ceases. When the pericarp, however, becomes suc- 

 culent whilst ripening, it assumes various tints ; transpiration 

 goes on from its outer cells, the contents of which thus become 

 dense, and absorb the watery matters from those within 

 them ; these in like manner react upon the contents of those 

 within them, and so there is a constant passage of fluid matters 

 from the surrounding parts by osmotic action into the peri- 

 carp ; in this way, therefore, it continues to enlarge, until it has 

 arrived at maturity, when transpiration nearly ceases from the 

 deposition of waxy matter in or upon the epidermal cells, and 

 the stalk by which it is attached to the plant becomes dried up. 

 When first formed such pericarps have a like composition with 

 leaves, and but little or no taste. After a time they acquire an 

 acid flavour from the formation of vegetable acids, and salts 

 with an acid reaction. The nature of these acids and salts 

 varies in different fruits; thus the Grape contains tartaric 

 a,cid chiefly and bitartrate of potash, the Apple, malic acid, and 

 the Lemon, citric acid. As the pericarp ripens, saccharine matter 

 is formed, and the quantity of free acids diminishes, partly from 

 their conversion into other matters, and partly from their combi- 

 nation with alkalies. In order that these changes may be pro- 

 perly effected, it is necessary that the fruit be exposed to the 

 sun and air, for if grown in the dark, it will continue acid : and it 

 will be much less sweet even when developed in difRised daylight, 

 than when freely exposed to the sun. As fruits ripen they 

 evolve carbonic acid gas, as already noticed, give off watery 

 fluids, and a sensible elevation of temperature may be noted. 



The origin of the sugar of fruits, and even its nature, is not 

 satisfactorily determined. According to most observers, ripe 

 fruits contain grape sugar, but M. Buignet states that the sugar 

 which is primarily formed in acid fruits is saccharose or cane 

 sugar (Ci2H22 0ji), and that during the process of ripening, 

 this sugar is gradually changed into fructose (Cg H^^ Og), but 

 very often there remains in the ripe fruit a mixture of these 

 two sugars. The origin of the sugar is variously attributed to 

 the transformation of the acids, cellulose, lignin, starch, dextrin, 



