56 THE ELEMENTARY STUUCTORE OF PLANTS. 



abounds in the almond, bean, and some other esculent seeds, in the 

 tubers of Orehises (as Salep, &c.), and forms the principal substance 

 of many sea-weeds, such as the Carragheen Moss (Chondrus crispus), 

 from which jelly is obtained for culinary jjurposes. When dry, it is 

 horny or cartilaginous, and lines the cells ; when moist, it swells 

 up, becomes gelatinous, and is capable of being perfectly diffused 

 through cold water. We have it as an excretion in Gum Traga- 

 canth. True gums, such as Gum Arabic, are states of nearly the 

 same substance, and are likewise formed only as excretions. 



84. Fixed Oils belong to the class of ternary products, but they 

 contain little oxygen, and some of them none at all. The fatty oils 

 take the place of starch in the seeds of many plants (as in flax-seed, 

 walnuts, &c.), and of sugar in some fruits, such as the olive. They 

 also occur in the herbage of most plants. 



85. Wax is a product of nearly the same nature as the fixed oils, 

 only it is solid at the ordinary temperature. It occurs as an excre- 

 tion, particularly on the surface of leaves and fruits, forming the 

 bloom or glaucous surface which repels water, and so prevents such 

 surfaces from being wetted. It forms a thick coating on some fruits, 

 as tlie bajberry. Wax also exists in all herbage, being one of the 

 components of the green matter of plants (92). 



86. Vegetable Aeids. Tartaric, Citric, and Malic Acids are the 

 principal kinds ; they are found in the herbage of those plants which 

 have a sour juice, such as Sorrel and the Grape- Vine. They are 

 ternary products, with a larger proportion of oxygen than starch, 

 sugar, and the like. They do not appear to play any leading part 

 in vegetation. They seldom exist in a free state, but are combined 

 with the allialoids, and with tlie inorganic or earthy alkalies (Potash, 

 Soda, Lime, and Magiaesia), which are introduced into plants from 

 the soil with the water imbibed by the roots. The more soluble 

 salts thus produced are found dissolved in the sap ; the more insolu- 

 ble are frequently deposited in the cells, either as an incrustation of 

 their walls, or in the fonn of minute crystals. When these crystals 

 contain a vegetable acid, it is almost always Oxalic Acid. This is 

 an almost universal vegetable product, and is a binary body (that is, 

 consists of two elements only, carbon and oxygen), differing from 

 carbonic acid in ultimate composition only in having a little more 

 oxygen. Hydrocyanic or Prussic Acid is one of the special pro- 

 ducts peculiar to certain plants, and of very different composition, 

 containing a large portion of nitrogen. 



