PECULIAR JUICES. 95 



Gum-arabic is obtained from the acacia of Arabia by 

 incisions made in the stem. 



Gum-tragacanth exudes naturally from the stem of 

 Astragalus. This secretion, which accumulates during 

 the night, when little or no evaporation takes place, 

 swells the wood which presses against the bark, and, this 

 dry coating not being susceptible of a similar distention, 

 the gum forces its way through it. 



Gum-resins appear to be a mixture of the two vegeta- 

 ble products from which their name is derived, and are 

 common to all umbelliferous plants. 



Manna is a saccharine secretion, which abounds in 

 the small-leaved ash of Calabria. It is to be found also, 

 in smaller quantities, in several other trees, such as the 

 larch and the willow. 



The essential or volatile oils bear a strong resemblance 

 to resins ; they are enclosed in small vesicles, whence 

 they are extracted by pressure. They are imprisoned, in 

 this manner, in the rind of the orange and the bark of 

 the cinnamon tree, in the wood of the sandal-tree, and 

 in a great variety of leaves, such as those of the geranium 

 and the orange, and in flowers of almost every descrip- 

 tion. In a word, there is scarcely any part of a plant 

 from which essential oil may not be extracted, excepting 

 the seed, from which it is absolutely excluded. 



Fixed oils, on the contrary, are almost exclusively con- 

 tained in the seed, where they constitute the most ap- 

 propriate nourishment for the embryo plant. There is, I 

 believe, but one exception- to this rule, the olive, where 

 the oil abounds not only in the seed, but also in the pulp 

 of the fruit, whence it is expressed to supply our tables, 

 or for the purpose of combustion. Nut-oil, linseed-oil, 

 and all the fixed oils, are not, like the essential oils, en- 

 closed in appropriate vessels, but are lodged in every in- 

 terstice of the seed. We shall speak more fully of this 

 when we come to examine the organization of this asylum 

 of the embryo plant. 



523. From what and how is Gum-arabic obtained! 524. What id 

 said of Gum-tragacanth! 525. Of what do gum resins appear to be 

 mixtures! 526. What is said of manna! 527. What said of 

 volatile oils, and in what are they contained! 528. In what are fixed 

 oils contained! 529. What exception is there! 590. What ft 

 said of nut-oil, linseed-oil, and of other fixed oils! 



