PLASTIDS AND CHONDRIOSOMES 



107 



taneously in a single amyloplast, later growing together to form :i 

 "compound grain " with more than one hilum. In case the parts making 

 up the compound grain are enveloped in one or more common outer 

 layers the grain is said to be "half-compound." Potato starch is made 

 up of simple, compound, and half-compound grains, whereas in oats 

 and rice all or nearly all of the grains are said to be of the compound type. 

 The successively deposited layers making up the grain differ mainly in 

 water content, the innermost layers being richest and the outermost 

 poorest in water. As a result of this non-uniform composition the grain 

 often splits radially when dried. 



' [ 



FIG. 39. Reserve starch grains from various plants. 

 A, potato; simple and half-compound grains. B, Colombo starch. C, arrowroot. 



D, pea. E, maize; intact and partially digested grains. F, rye. G, maize. 

 /, bean. J, rice. K, wheat. (After Tschirch.) 



H, Euphorbia. 



As a result of his classic researches Nageli (1858) advanced the theory 

 that the starch grain is made up of ultramicroscopic crystalline particles 

 which he called " micellae/' these being surrounded by water films of 

 varying thickness. It was similarly held by A. Meyer (1883, 1895) 

 that the grain is composed of radially arranged needle-shaped crystals 

 known as "trichites;" these are composed of a- and /3-amylose which 

 turn blue with iodine. In some starch amylodextrin and dextrin are 

 also present, such grains turning red with iodine. Both Nageli and Meyer 

 held the stratification of the grain to be due to the varying numbers of 

 the crystalline units in the successive layers, and Meyer showed that in 

 certain cases it is correlated with the alternation of day and night, and 

 therefore with a periodic activity on the part of the plastid. This con- 

 clusion was confirmed by Salter (1898). 



The statement made by Schimper (1880) and Meyer (1883, 1895) that 

 starch is always formed by plastids still holds good in its essential feature : 

 so far as is certainly known no primary product of photosynthesis is 

 formed in the cytoplasm apart from plastids, although in some cases, 

 such as the young cells of Hydrodictyon, according to Harper, it is very 

 difficult or even impossible to distinguish the limits of these organs. The 



