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INTRODUCTION TO CYTOLOGY 



been held by some observers that the stroma is a homogeneous body in 

 which the droplets of chlorophyll solution are imbedded, and that the 

 reticular structure so often reported is an artifact due to the reagent 

 employed in removing the chlorophyll. By others the pigment has been 

 thought to form a layer about the plastid. In any case it seems evident 

 that the chlorophyll is not uniformly distributed throughout the stroma. 

 In chromatophores other than chloroplasts the pigments may at times 



take the form of solid granules or crystals. 



Starch. — After a period of photosynthetic 

 activity the chloroplast contains starch, the first 

 visible product of that activity, in the form of 

 minute granules. This "assimilation starch" is 

 formed within the body of the chloroplast, as 

 Meyer originally showed (Fig. 38, A, B). It is 

 later transformed through the agency of enzymes 

 into some soluble compound, usually a sugar; in 

 this form it may be carried to growing regions, 

 where, after further changes, it is built into the 

 structure of the plant. Or, it may pass to storage 

 organs where it is transformed into the ordinary 

 "reserve starch," or "storage starch." This de- 

 position of reserve starch is brouj; ht about through 

 the agency of a?nyloplasts, which are leucoplasts 

 capable of changing already elaborated organic 

 materials, such as glucose, into starch (Fig. 38, C). 

 Reserve starch, upon which we depend so 

 largely for food, is a carbohydrate with a composi- 

 tion expressed by the general formula (C 6 Hio0 5 )n, 

 and exists in the form of granules ranging in size 

 approximately from 2//. to 200//, in different plants. 

 Potato-starch grains are usually about 90/.; in 

 diameter. The reserve starch grain is formed 

 within the body of the amyloplast, and is made 

 up of a series of concentric layers successively 

 laid down about a center, or "hilum" (Fig. 39, A). 1 In case the grain 

 starts to form near the middle of the amyloplast it may develop sym- 

 metrically, but commonly the developing grain lies near the periphery 

 of the amyloplast, which becomes greatly distended as the grain grows. 

 Material is thus deposited unevenly upon the grain so that the latter 

 becomes very eccentric; in extreme cases the grain ruptures the amylo- 

 plast and remains in contact with it only at one side, where all new 

 material is then deposited. Several grains may start to develop simul- 



1 For the structure of the starch grain see the papers of Nageli, Schimper, Meyer, 

 Binz, Dodel, Salter, and Kramer. 



Fig. 38. — Formation of 

 starch by plastids. 



A, dividing chloro- 

 plasts of Funaria, with 

 grains of assimilation 

 starch. X 940. (After 

 Strasburger.) B, chloro- 

 plast of Zygnema, with 

 several large starch 

 grains about a central 

 pyrenoid. (After Bour- 

 quin, 1917.) C, leuco- 

 plast (amyloplast) in 

 aerial tuber of Phajus 

 grandifolius with grain 

 of reserve starch. (After 

 Strasburger.) 



