VEGETABLE PRODUCTS. STARCH. 141 



closing a soluble substance. By boiling in water, the pellicle bursts, 

 and the contents are dissolved, becoming gelatinous on cooling. The 

 circular markings and strias seen on the grains, and the part called the 

 hilum, have already been noticed (If 17). Some plants, such as 

 potato, arrow-root, and wheat, contain a large quantity of starch, 

 which varies, however, in quantity according to the period of growth. 

 Thus, while starch abounds towards the latter part of the season in the 

 potato, it decreases when the tubers begin to germinate in spring. 

 It was found that 240 Ibs. of potatoes, left in the ground, contained 

 of starch : 



In August, 23 to 25 Ibs., or 9 '6 to 10'4 per cent. 



September, 32 



October 32 



November, 38 



April, 38 



May, 28 



38 " " 13-3 " 16 



40 " " 13-3 " 16-6 



45 " " 16 " 187 



28 " " 16 " 11-6 



20 " " 11-6 " 8-3 



The quantity of starch remained the same during the dormant state 

 hi winter, but decreased whenever the plant began to grow, and to 

 require a supply of nourishment. 



302. Starch is stored up hi many seeds. It exists hi roots, especially 

 hi those which are fleshy; hi stems; hi the receptacles of flowers; and 

 hi pulpy fruits. The seed-lobes of the Bean and Pea, and many other 

 leguminous plants; the roots and the under-ground stem of Maranta 

 arundinacea or Arrow-root, and of Canna coccinea or Tous-les-mois, 

 Canna Achiras and C. edulis; the stem of the Sago Palm (Sagus 

 Rumphii and farmifera), and of the Cycas tribe; the receptacle of the 

 artichoke, and the pulp of the apple, are familiar instances of parts 

 in which starch abounds. The grains of potato-starch are pearly 

 or sparkling hi their appearance, of large size, having one or more 

 hila, and often cracks on the surface. Those of arrow-root are dull, 

 white, and small, while those of Tous-les-mois, present a glistening 

 appearance like potato-starch, and are larger. In some cases, starch 

 is associated with poisonous or acrid juices, as in Jatropha Manihot, 

 which yields Cassava and Tapioca, and hi Arum maculatum, the under- 

 ground stem of which furnishes Portland sago. Inuline is a substance 

 analogous to starch, found hi the roots and tubers of Inula Helenium 

 (Elecampane), Dahlia variabilis, and Helianthus tuberosus (Jerusalem 

 artichoke); while Lichenin is a variety of starch occurring hi Cetraria 

 islandica (Iceland moss). Lichenin or lichen starch consists of C 12 H 10 

 O 10 , and is deposited hi the primary cell- wall of the plant, hi the form 

 of an incrusting layer. By the action of malt or of sulphuric acid 

 upon starch, or by long boiling in water, a gummy matter is pro- 

 duced called dextrin* or soluble starch composed of C 12 H 10 O 10 . Some 



* Dextrin is so called from possessing the property of effecting the right-handed rotation of the 

 plane of polarization of a ray of light. 



