•SKCT. I 



MORPHOLOGY 



65 



from the centre. The disc-shaped starch grains of wheat are of 

 unequal size, and only indistinctly stratified. A comparison of the 

 accompanying figures (Figs. 65-67), all equally magnified, will give 

 an idea of the varying size of the starch grains of difterent plants. 

 The size of starch grains 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. 65 £) and compound starch grains 

 (Fig. 65 C, D) occur in potatoes, together 

 with simple grains. In oats (Fig. 67) 

 and rice all the starch grains are com- 

 pound. According to Nageli ('*''), the 

 compound starch grains of rice consist 

 of from 4 to 100 single grains; those 

 of the oat of about 300, and those 

 of Spinacia glabra sometimes of over 

 30,000. Starch thus formed from pre- 

 viously assimilated organic substances 

 also requires chromatophores for its 

 production. The grains are formed 

 by means of leucoplasts, which are, in 



D 



Fig. OS. — Leucoplasts from an aerial tuber 

 of I'hajus grandifolius. A, C, D, E, 

 viewed from the side ; B, viewed from 

 above ; E, leucoplast becoming green 

 and changing to a cliloroplast. (X 

 540.) 



consequence, often termed starch- 

 builders. If the starch grain is uni- 

 formly surrounded by the leucoplast 

 during its formation it grows uniformly on all sides and is symmetrical 

 about its centre. If the formation of a starch grain begins near 

 the periphery of a leucoplast, the grain will grow more rapidly on the 

 side on which the main mass of the leucoplast is present and the 

 starch grain thus becomes excentric (Fig. 68). Should, however, 

 several starch grains commence to form at the same time in one 

 leucoplast, they become crowded together and form a compound starch 

 grain, which, if additional starchy layers are laid down, gives rise 

 to a half-compound grain. 



It has been asserted that starch grains are crystalline bodies, so-called 

 sphaerites or sphaero-crystals {'^'^), and are composed of fine, radially arranged, 

 needle-shaped crystals, which A. Meyer terms trichites. Their stratification, 

 according to this view, is due to variations in the form and number of the crystal 

 needles in the successive layers. On the other hand, H. Fischer (-i^) has explained 



