44 INULIN 



Palms (mainly species of Metroxylon, cultivated in the Malay 

 region), from which it is extracted after the appearance of the 

 inflorescence. 



The wide distribution of starch as a form of storage of car- 

 bohydrate material can probably be related to its insoluble 

 character, in consequence of which it exerts no osmotic pressure. 

 The small amount of moisture in seeds renders them unsuited 

 to the storage of soluble carbohydrates, and doubtless explains 

 the frequent occurrence of starch in the endosperm and cotyle- 

 dons. In succulent storage organs, on the other hand, soluble 

 carbohydrates often occur. One of the most important is inulin, 

 another of the complex polysaccharides, though with a molecular 

 weight smaller than that of starch. As a result, in spite of its 

 solubility and frequent high concentration, inulin has but little 

 effect on the osmotic pressure of the cells containing it (cf. p. n). 



Inulin is found especially in the Compositae and the allied 

 family of the Campanulaceae, but also in the bulb of the Wild 

 Hyacinth (S cilia nutans) and in other Monocotyledons. Soluble 

 carbohydrates of a similar chemical constitution are, moreover, 

 encountered in many members of the latter group (e.g. the gra- 

 minin of Grasses, the irisin of the Iris, etc.). 



In sections of a fresh Dahlia-tuber, Salsify-root, or Jerusalem 

 Artichoke, the intact parenchymatous cells will be seen to contain 

 a faintly yellow oil-like liquid, the inulin. On placing the sections 

 in spirit, the inulin is deposited as a finely granular precipitate. 

 In sections of material kept for some weeks in spirit, so that 

 the latter has only penetrated slowly into the tissues, the inulin 

 will be found as big spherical or lobed masses deposited on the 

 cell-walls. These sphere-crystals (Fig. 21, In.) usually show con- 

 centric layers, whilst radial lines traversing them indicate the 

 numerous needle-like units of which they are built up. On 

 warming in water, the precipitates and sphere-crystals of inulin 

 readily dissolve. Iodine gives a brown colouration with inulin 

 which is but little different from that of the reagent itself. 



Sugars are among the most important of the soluble carbo- 

 hydrates present in plants. They possess much simpler molecules 

 than the polysaccharides just considered, being either mono- 

 saccharides with the general formula C 6 H 12 O 6 , or disaccharides 

 with the formula C^H^On. Disaccharides and polysaccharides 



