20 CHEMISTRY OF PLANTS. 



pensable in an investigation of this kind. His results form the second 

 hypothesis upon the nature of starch, which we may oppose as the views 

 of German vegetable physiologists against those of the French chemists. 

 According to the former, starch is composed of layers ranged over one 

 another, all consisting of the same chemical substance. The external 

 layers are less easily soluble in water, owing to their saturation with 

 foreign substances. In the interior there is an extremely small nucleus, 

 which appears, by its behaviour during the action of hot water, acids, and 

 alkalies, neither to be starch, gum, nor sugar. This is more especially the 

 case with potato starch, butthe starch granules of the curcuma roots appear 

 to differ somewhat from this, exhibiting elongated flat discs, while those 

 of the cereals have lenticular bodies. Subsequent observation (especially 

 on the part of Meyen) has shown abnormally-irregular forms in the milky 

 juice of the Euphorbiacece. And here the theory rests, as far as the 

 main point is concerned. There has as yet been no thought of a more 

 exact investigation of the structural signification of the chemical rela- 

 tions, or of a comprehensive comparison of the different kinds, of starch in 

 various plants. The whole of this question has been condensed in 

 Endlicher and Unger in the following improper manner : " Amylum 

 granules consist of a more or less solid (?) nucleus, around which layers 

 of solid (!) consistency are by degrees eccentrically deposited, admitting 

 sometimes even of being peeled off. (?) On the external case(?) of the 

 ainyhum granule being burst, the interior will dissolve even in cold 

 water, and that about 0*413 of the whole granule. The chemical cha- 

 racter of the nucleus is not essentially different from that of the layers 

 which either partially or entirely invest it. (!) Iodine colours both parts 

 in like manner, blue ! Concentrated (?) mineral acids dis- 

 solve the arnylum granule, boiling water occasions only an enlargement 

 in its size by means of absorption, and this often gives rise to a cleft in 

 the external layers (!) through which the softer nucleus (!) is expressed. 

 The probable special substance of the nucleus, the so-called dextrin, 

 consists of gum and sugar." (!!!) 



By way of historical references we may recommend Poggendorff, 

 Annalen, vol. xxxvii. (1836), p. 123. ; Meyen's Physiologic, p. 190. ; Mul- 

 der's Physiol. Chem. Moleschott, p. 215.* 



5. Gum (Arabin, Dextrin, Vegetable Mucilage in part). In a pure 

 state it is clear; when dry, brittle like glass; easily soluble in water, and 

 also in dilute acids; not soluble in ether, alchol, and volatile and fixed 

 oils. The action of alcohol makes it horny, and it is coloured pale- 

 yellow by iodine. It passes through cerasin and some so-called varieties 

 of mucilage into vegetable jelly; it borders through dextrin on starch. 

 The analysis of gum Arabic by Berzelius gives the formula, C 12 H 11 

 Oil; of gums Arabic, Senegal, and Java, by Mulder, C 12 H 10 O 10. 



It is found in a state of solution in the interior of cells, or as a secre- 

 tion in the great gum canals, and not unfrequently mixed with vegetable 

 jelly, and is frequently, through foreign substances, coloured yellow or 

 brown, a condition in which it is almost always found when collected for 

 commercial purposes. Some groups of plants are distinguished by the 

 great quantity of gum they produce, as the Mimosece and the Cycadece. 



The substance called dextrin, and which can be formed through the 

 action of dilute sulphuric acid, diastase, &c., on cellulose or starch, 

 agrees with gum in many points, and especially in its elementary com- 

 position. It seems to be a substance of more importance than gum. 

 According to Mulder, the greater part of what has hitherto been called 



