408 Vegetable Physiolorjy. 



similar substance, ?'. e. cellulose metamorphosed structurally and 

 chemically into a homogeneous substance intermediate between 

 cellulose and dextrine, insoluble in water, but naturally containing 

 a great quantity, and passing into a horny condition if dried. 



Inulin and pectin are products which have been described and 

 studied by chemists, and their relations with starch and the 

 soluble ternary compounds seem very close. The microscope is, 

 however, of little avail in the investigation of these substances, 

 since they present no definite structure as they exist in nature. 

 Inulin is extracted from the tubers and tuberous roots of various 

 plants of the botanical order Compositsp, such as that of the 

 Jerusalem artichoke {HeliMithus tuberosus), the root of elecam- 

 pane, the dahlia, &c. It is obtained separate by extraction with 

 boiling water, which in cooling deposits a structureless granular 

 substance, which is not coloured blue by iodine. Its composition 

 is the same as that of cellulose, and it is very readily converted 

 into sugar ; hence it would appear to be a substitute for starch 

 and dextrine peculiar to certain plants. 



Pectin is obtained as a gelatinous matter from various fruits, 

 -such as apples, pears, Arc, and from roots, such as the turnip and 

 <;arrot, by boiling, and its occurrence in large quantities in the 

 economical plants imparts to it a high claim to the attention of 

 physiologists. Unfortunately the chemical part of its history 

 still presents a somewhat complicated and uncertain collection of 

 results, which are incapable of being brought into relation with 

 the other facts of the nutrition of plants. The analysis of 

 pectin shows that it differs from the starch and cellulose series in 

 not containing oxygen and hydrogen in the proportions of water. 

 It is found in ripe fruits, with sugar, where vegetable acids have 

 been previously abundant, and it is said to be produced from 

 imripe fruits by boiling with sulphuric or malic or tartaric acids. 

 Now as the cell-walls of the succulent tissues of fruit are some- 

 what hard and resisting before ripening, and afterwards become 

 very delicately membranous, the substance (outer lamellae of the 

 cell-wall ?) originally cementing them together dissolving away, 

 pectin would appear to be derived from the action of acids 

 upon cellulose ; hence it approaches in its mode of origin to 

 <lextrine, but the chemical composition opposes an obstacle to 

 any further comparison between them. The investigation of the 

 subject of pectin, especially in connexion with the growth of 

 turnips and mangel wurzel, is one of the pressing desiderata of 

 agricultural science. 



The further constituents of the watery cell-sap are to be 

 regarded as rather special than general, for such of them as occur 

 indiscriminately in all parts of plants are rather vmassimilated 

 products carried forward in the diffusion of the sap than proper 



