66 BOTANY tart i 



the stratification as due to zonal splits rich in water, which originate by contraction 

 taking place in the substance of the grain at some distance from its growing 

 surface. Starch grains are composed of a carbohydrate, the formula of which is 

 (CgHj„05)n. Most starch-grains consist of amylose, and are coloured blue with 

 iodine and with further treatment almost black. Tlie grains swell in water at 

 GO"-?^ C, are but little soluble in water at 100° C, but are rapidly dissolved by 

 super-heated water at 140°-150° C. Starch swells very readily at ordinary 

 temperatures in solutions of caustic potash or soda. Heated without addi- 

 tion of water, i.e. roasted, starch becomes transformed into dextrin, and is 

 then soluble in water and correspondingly more digestible. According to 

 Maquenne and Roux (^°) and to Gatin Ghuzewska ("i) the starch grain consists 

 mainly of amylose, which stains blue with iodine, and of a lesser proportion of 

 amylopectin which is coloured violet-blue by iodine ; amylopectin is related to 

 glycogen. The inner portion of the grain of potato starch consists of amylose, 

 while the amylopectin forms a peripheral layer. The glutinous starch of varieties 

 of Rice and Millet {Oryza saliva, var. glutinosa, Sorghum vulyare, glutinosum) is 

 in the completely unswollen condition coloured brown with iodine ; when swollen 

 by a dilute solution of iodine, it takes a red, wine-red, or purple colour. Accord- 

 ing to 0. BtJTSCHLi (^^) it consists of aniyloerythrin, a carbohydrate resembling 

 starch. In polarised light, starch grains, like inorganic sphaerites, show a dark 

 cross. This appearance is usually referred to the doubly refractive nature of the 

 elements of which the starch grain is constructed. 



The amount of starch contained in reservoirs of reserve material 

 is often considerable ; in the case of potatoes 25 per cent of their 

 whole weight is reserve starch, and in wheat the proportion of starch 

 is as high as 70 per cent. The starch flour of economic use is derived 

 by washing out the starch from such reservoirs of reserve starch. In 

 the preparation of ordinary flour, on the contrary, the tissues contain- 

 ing the starch are retained in the process of milling. 



Aleurone. — Aleurone or proteid grains are produced in the seeds 

 of numerous plants, especially in those containing oil. They are 

 formed from vacuoles, the contents of which are rich in albumen, 

 and harden into round grains or, sometimes, into irregularly shaped 

 bodies. The alljuminous substances of which they consist are mainly 

 globulins (•''^). A portion of the albumen often crystallises, so that 

 frequently one and occasionally several crystals are formed M'ithin the 

 aleurone grain. In aleurone grains containing albumen crystals there 

 may often be found globular bodies termed globoids, which consist 

 of a double phosphate of magnesium and calcium in combination with 

 globulins (^■*). Crystals of calcium oxalate are also found enclosed 

 in aleurone grains. Free globoids are found in the cytoplasm of 

 some seeds. 



The seeds of Ricinus (Fig. 69) furnish good examples of aleurone grains witli 

 enclosed albumen crystals and globoids. The aleurone grains themselves lie 

 embedded in a cytoplasm that is rich in oil. In the cereals the aleurone grains, 

 which lie onl}' in the outer cell layer of the seeds (Fig. 70 «V), are small, and free 

 from all inclusions ; they contain neither crystals nor globoids. As tlie outer 

 cells of wheat grains contain only aleurone, and the inner almost exclusively 



