60 THE ELEMENTARY STRUCTURE OF PLANTS. 



in part) has the chemical composition and nearly the properties of 

 starch after it has been diffused in hot water. It is not only one 

 of the contents of cells, as in the tubers of Orchises, in many 

 fruits, &c., and largely in those of Alga?, but it also forms in great 

 part the cell-wall of Alga3, as in the Carragheen Moss (Chondrus 

 crispus), from which vegetable jelly is obtained for culinary pur- 

 poses. When dry, it is horny or cartilaginous ; when moist, it 

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

 perfectly through cold water. It passes by various modifications, 

 on the one hand into cellulose, and on the other into starch and 

 dextrine. We have it as an excretion in Gum Tragacanth. True 

 gums, such as Gum Arabic, &c., are altered states of the same 

 substance, or of dextrine, and are likewise formed only as ex- 

 cretions. 



84. Sugar (of which there are two distinct kinds, Cane and Grape 

 Sugar) is the most soluble of the many forms of ternary organiza- 

 ble matter, as already stated. Though sometimes crystallized as 

 an excretion in the nectaries of flowers, yet in the plant it exists 

 only in solution. It abounds in growing parts, and in pulpy fruits. 



85. Fixed Oils belong to the class of ternary assimilated products, 

 but they contain little or no oxygen. The fatty oils take the place 

 of starch (from which they are probably formed) in the seeds of 

 many plants (as in flax-seed, walnuts, &c.), and of sugar in some 

 fruits, such as the olive. They also exist in the herbage, and in 

 some smaller proportion in the cells, perhaps, of almost all plants. 



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

 only it is solid at the ordinary temperature, which is extensively 

 found in plants as an excretion, 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 

 exists largely on some fruits, as the bayberry. As bees convert 

 sugar into wax, and as the sugar-cane yields a kind of wax which 

 " sometimes passes into sugar," we may infer that wax is formed 

 in the vegetable of sugar or its kindred products. Wax also exists 

 as one of the contents of cells, of leaves especially, where it ap- 

 pears to form the basis of 



87. Chlorophyll, the substance which* gives the universal green col- 

 or to the leaves and herbage. It is formed only in parts exposed 

 to the light, such as the green bark, and especially the leaves ; 

 not, however, in the external layer of cells, or epidermis (69), but 



