70 BOTANY part i 



on the side remaining in contact with the plastid, the starch grain 

 thus becomes eccentric (Fig. 77). Should, however, several starch 

 grains commence to form at the same time in one leucoplast, they 

 would become crowded together and form a compound starch gram, 

 which, if additional starchy layers are laid down, gives rise to a half- 

 compound grain. 



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

 sphserites, and are composed of fine, radially arranged, needle-shaped crystals 

 (trichites). Their stratification, according to this view, is due to variations in the 

 form and number of the crystal needles in the successive layers. In a few 

 individual cases, Arthur Meyer has succeeded in showing that the stratification 

 of the starch grains corresponds to the alternation of the periods of day and night, 

 i.e. to the interference which is thus eaused'in the nutritive processes. The growth 

 of starch grains is also affected by the solvent action of surrounding substances, 

 whereby the peripheral layers may be partially removed, and then no longer com- 

 pletely envelop the entire grain. Starch grains are composed of a carbohydrate, 

 the formula of which is (C 6 Hio0 5 )n. Most starch grains only contain amyloid, 

 one variety of which becomes liquid in the presence of water at a temperature 

 of 100° C, and another, which, under the same conditions, does not become liquid. 

 In addition to this amyloid many starch grains contain also amylodextrin. In 

 certain cases, as in Orym sativa var. glutinoso, and Sorghum vulgare var. glutinosnm, 

 the starch grains consist principally of amylodextrin. Although starch rich in 

 amyloid gives a blue reaction with a solution of iodine, the starch rich in amylo- 

 dextrin takes a red wine colour. Starch grains become swollen in water at a tem- 

 perature of 60° to 70° C, according to Arthur Meyer, because of the conversion 

 into tenacious globules of the more readily soluble of the two amyloids ; at 138° C. 

 starch grains become completely dissolved. Starch swells very readily at ordinary 

 temperatures in solutions of potassium, or sodium hydrate. Heated without addi- 

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

 soluble in water and correspondingly more digestible. That starch grains give a 

 dark cross in polarised light is due to the double refraction of the component 

 crystalline elements. 



The amount of starch contained in reservoirs of reserve material is often con- 

 siderable ; 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 

 j-eserve starch. In the preparation of ordinary flour, on the contrary, the tissues 

 containing the starch are retained in the process of milling. 



Aleurone. — Aleurone or protein grains (gluten) 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 irregular 

 bodies of indefinite shape. A portion of the albumen often crystal- 

 lises, so that frequently one and occasionally several crystals are 

 formed within one aleurone grain. In aleurone grains containing 

 albumen crystals there may often be found globular bodies, termed 

 globoids, which, according to Pfeffer, consist of a double phosphate 

 of magnesium and calcium in combination with some organic substances. 



