64 



BOTANY 



PAKT I 



crystals, various substances which are elsewhere not crystalline, and 

 the cell-sap. 



Starch. — The chloroplasts in plants exposed to the light almost 

 always contain starch grains. These grains of starch found in the 

 chloroplasts are the first visible products of the assimilation of 

 inorganic matter. They are formed in large numbers, but as they 

 are continually dissolving, always remain small. Large starch grains 

 are found only in the reservoirs of reserve material, where starch is 

 formed from the deposited products of previous assimilation. Such 

 starch is termed reserve starch, in contrast to the assimilation 

 STARCH formed in the chloroplasts. All starch used for economic 



Flo. 60. — Starch grains from the 

 cotyledons of PItdseolns ml- 

 garis. (x 540.) 



t; 



Fio. 65.— Starch grains from a potato. A, sinijile ; 

 B, half-compound ; C and D, compound starch 

 grains ; c, organic centre of the starch grains, or 

 nucleus of their formation, (x 540.) 



Fig. 67. — Starch grains of the oat, 

 Avena sativa. A, Compound 

 grain ; B, isolated component 

 grains of a compound grain, 

 (x 540.) 



purposes is reserve starch. The starch grains stored as reserve 

 material in potatoes are comparatively large, attaining an average size 

 of 0-09 mm. As shown in the above figure (Fig. 65^^), they are 

 plainly stratified. The stratification is due to the varying densities 

 of the successive layers ; thicker denser layers which appear clear by 

 transmitted light alternate with thinner less dense layers which 

 appear dark. They are excentric in structure, since the organic 

 centre, about which the diflierent layers are laid down, does not 

 correspond with the centre of the grain but is nearer to one margin. 

 The starch grains of the leguminous plants and cereals, on the other 

 hand, arc concentric, and the nucleus of their formation is in the 

 centre of the grain. The starch grains of the kidney bean, Phascolm 

 vuUjaris (Fig. 66), have the shape of flattened spheres or ellipsoids ; 

 they show a distinct stratification, and are crossed by fissures radiating 



