310 THE PEUIT. 



587. Ripening. The pericarp sometimes remains herbaceous in 

 texture, hke the pea-pod, or becomes thin, dry, and membranaceous, 

 hke the pod of the Bladder-Senna. In such cases it is furnished 

 with stomates, continues to have chlorophyll in its cells, and acts 

 upon the air like an ordinary leaf. In other plants the pericarp 

 thickens, and either becomes hai-d and dry, like a nut, or else fleshy 

 or pulpy, like a berry (gooseberry, grape, &c.). Sometimes the 

 outer portion softens into flesh or pulp, while the inner portion hard- 

 ens, thus forming a stone-fruit, like the cherry and peach. 



587'. Most fleshy or pulpy fruits are tasteless or slightly bitter 

 during their early growth ; at which period their structure and 

 chemical composition are similar to that of leaves, consisting of cel- 

 lular with some woody tissue ; and their action upon the atmosphere 

 is likewise the same (346). In their second stage, they become 

 sour, from the production of acids (353) ; such as tartaric acid in 

 the grape ; the citric, in the lemon, orange, and the cranberry ; the 

 malic, in the apple, gooseberry, &c. At this period they exhale very • 

 little oxygen, or even absorb that substance from the surrounding 

 air. The acid increases until the fruit begins to ripen, when it grad- 

 ually diminishes, and sugar is formed. In the third stage, or that of 

 ripening, the acids, as well as the fibrous and cellular tissues, gradu- 

 ally diminish as the quantity of. sugar increases ; the latter being 

 produced partly at the expense of the former. A chemical change, 

 similar to that of ripening, takes place when the green fruits are 

 cooked ; the acid and the mucilaginous or other products, by the aid 

 of heat reacting upon each other, are both converted into sugar. 

 Mingled with the saccharine matter, a large quantity of vegetable 

 jelly (83) is also produced in most acidulated pulpy fruits, ex- 

 isting in the form of pectine and pectic acid. These arise from 

 the reaction of the vegetable acids during ripening upon the dex- 

 trine and other ternary products accumulated in the fruit. 



588. When the walls of a pericarp form two or more layers of 

 dissimilar texture, the outer layer is called the Epiearp, the middle 

 one, Mesocarp, and the innermost, Endocarp. A stone-fruit or 

 drupe, like the peach, consists of two layers, viz. the outer or fleshy 

 layer, which is therefore termed the Sarcocarp, and the inner, or 

 endocarp, the shell or stone, which is also termed the Putamen. 



589. Fruits also may be divided into the indehiscent or closed, and 

 the dehiscent or those that open. Fleshy fruits generally, stone- 

 fruits, and many dry fruits, especially one-seeded ones, such as nuts, 



