34 BOTANY PART i 



two very different sizes, and only indistinctly stratified. In addition 

 to the simple starch grains so far described, half-compound and com- 

 pound 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. 25 B) and 

 compound starch grains (Fig. 25 C, D) occur in potatoes, together with 

 simple grains. In oats (Fig. 27) and rice all the starch grains are 

 compound. The compound starch grains of rice consist of from 4 to 

 100 single grains; those of the oat of about 300, and those of 

 Spinada glabra sometimes of over 30,000. Starch grains have thus 

 distinctive forms in different plants. 



The structure of starch grains becomes intelligible in the light of 

 their mode of formation. If the starch grain is uniformly 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. 28). 

 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. 



Starch grains are composed of a, carbohydrate with the formula 

 (C 6 H 10 5 ) n . When it is to be employed further in the metabolism of 

 the plant, starch is again transformed into sugar by the action of an 

 enzyme called DIASTASE. 



Starch grains may be regarded as crystalline structures, sphaero- crystals, or 

 sphaerites, which are built up of radially arranged, needle-shaped crystals of a- and 

 /3-amylose. With polarised light they show, like inorganic sphaerites, a dark cross, 

 an appearance depending on the doubly-refractive nature of the elements of the 

 starch grain. The stratification may be the expression of differences in form and 

 abundance of the crystalline needles in the successive layers. 



Starch grains are as a rule coloured, first blue and then almost black, by a 

 watery solution of iodine ; the grains of glutinous rice, however, stain wine-red, 

 possibly consisting of amylodextrine. They are easily swollen at ordinary 

 temperatures in solutions of potash or soda and by chloral hydrate. They also 

 swell and form a paste in water at 70-80 C. They dissolve, i.e. are transformed 

 into sugar without previous swelling, in concentrated sulphuric acid. Heated 

 without the addition of water, or roasted, the starch is transformed into an 

 imperfectly known substance that is soluble in water. 



IV. THE CELL WALL ( 30 ) 



Each protoplast in plants is as a rule enclosed by a firm invest- 

 ment called the cell wall. This is formed on the outside of the 



