74 



I'.OTAXY 



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 different layers are laid down, does not correspond with the centre 

 of the grain. The starch grains of the leguminous plants and cereals, 



FIG. 70. Starch grains from a potato. A, simple ; 

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

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

 nucleus of theii formation, (x MO.) 



KKI. So. Starch grains 

 from the cotyledons <>f 

 Ph<i.--< -. ( x 



540.); 



on the other hand, are concentric, and the nucleus of their formation 

 is in the centre of the grain. The starch grains of the Kidney Bean, 

 Phaseohis vulfjaris (Fig. 80), have the shape of a flattened sphere or 

 ellipsoid ; they show a distinct stratification, and are crossed by 

 fissures radiating from the centre. The 

 disc-shaped starch grains of wheat are of un- 

 equal size, and only indistinctly stratified. 

 A comparison of the accompanying figures 

 (Figs. 79-81), all equally magnified, will give 

 an idea of the varying size of the starch grains 



of different Pknts. The size of starch V* 

 varies, in fact, from 0'002 mm. to 0*170 mm. 



Starch grains 0'170 mm. large, such as those 

 from the rhizome of Canna, may be seen even 

 with the naked eye, as minute bright bodies. In addition to the simple 

 starch grains so far described, half-compound and compound starch 

 grains are often found. Grains of the former kind are made up of two 

 or more individual grains, surrounded by a zone of peripheral layers 

 enveloping them in common. The compound grains consist merely 

 of an aggregate of individual grains unprovided with any common 

 enveloping layers. Both half-compound (Fig. 79 ) and compound 



pound grain; B, isolated 

 comjionftnt grains of a com- 

 pound grain, (x ">4o.) 



