STARCH-GRAINS 43 



Whilst most of tho grains of Potato-starch show but a single 

 hilum, an occasional one will be found to possess two or three, 

 each with its own system of laj'ers (Fig. 20, c). This results from 

 the development of several grains wthin the same leucoplast, 

 growth of each occurring independent^ until they meet ; in 

 some cases deposition of starch continues ^^•ith the formation of 

 laj'ers common to the whole group (Fig. 20, t'). Such structures 

 are called compound grains. In some plants {e.g. in man}' cereals) 

 numerous grains arise in each leucoplast, so that the compound 

 structure may consist of a thousand or more units. Thus a 

 starch-grain of the Rice (Fig. 20, h) or Oat is marked out into 

 a number of small areas, each representing a constituent unit. 



Starch is a carbohydrate belonging to the group known as 

 polysaccharides — that is to say, its highh' complex molecule is 

 built up of CoHioOj groups present in larger numbers than 

 in the case of cellulose (cf. p. 31), but here as there the actual 

 number is unknown. The grains are regarded b\' some as con- 

 sisting of a number of alternating amorphous lavers, while others 

 believe them to possess a crj'stalline structure. Under polarised 

 light the}' exhibit a black cross with the hilum as its centre. 

 WTien warmed in a drop of water starch-grains swell, lose their 

 pronounced stratification, and ultimatelv pass into a " solution " 

 similar to that obtained when a thin starch-paste is treated with 

 an excess of boihng water. This "solution" is, however, col- 

 loidal in character, and therefore wiU not diffuse through an 

 organic membrane. A similar result is obtained when starch 

 is treated with caustic potash. Comparatively few dj'es colour 

 starch, although gentian violet and eosin are notable exceptions. 



The accumulation of starch-reserves by plants is a feature 

 of the greatest economic importance, constituting as they do a 

 very important article of human diet. The cereals, the pulses, 

 and Potatoes, all of which contain a high percentage of starch 

 (cf. p. 53), furnish a sufficient illustration. Rice, which perhaps 

 constitutes the most widely consumed food in the world, is the grain 

 of Oryza sativa, a Grass cultivated in most parts of the Tropics 

 where the necessarj' water for submerging the rice-fields is 

 available. Tapioca is obtained from the root-tubers of the 

 Cassava or Mandioc [Manihot utilissima, \\'idely grown in the 

 Tropics), whilst sago is the starch found in the pith of various 



