32 DIFFERENTIATION AND SPECIFICITY OF STARCHES. 



Schimper states that the processes of formation of starch-grains were very similar in 

 the several plants studied. The most important stages in the development he gives as 

 follows: (1) the appearance of starch-grains in the form of strongly refractive bodies; 

 (2) the differentiation of the originally homogeneous grain into a central watery hilum and 

 a dense peripheral layer; (3) in later stages tlie surrounding of the hilum by three layers, 

 the middle always being watery (a watery layer never occurs at the outside, and such a 

 layer probably originates by an alteration of an original dense layer) ; (4) the number of 

 layers increases, and the outer is always the most dense; (5) the watery content of the 

 inner parts of the grain increases with the increase in the volume of the grain. 



Schimper found that pressm-e on starch-grains causes the formation of numerous 

 cracks, which in simple grains are perpendicular, but never parallel to the lainellaj. The 

 cohesion of the starch-grains varies in a very striking manner with the line of direction 

 in which the pressure or strain is exerted — in a tangential direction it is very small, but 

 in a radial direction it is great. In the tangential direction he holds that there is no elas- 

 ticity of the substance, whereas in the radial direction it is marked. 



Schimper refers to the observation of previous investigators that in the swelling of 

 starch-grains in water the water is stored much more abundantly in a direction parallel 

 than perpendicular to the lamellae, and he states that one reason for this is that the cracks, 

 originating during the drying of starch, always run at right angles to the lamelliB, and that 

 cracks would also appear in other directions if the water were distributed equally in the 

 grains. The fact that the water is stored in larger quantities in a tangential than in a 

 radial direction causes tension in the grains. The grain is assumed to consist at first of a 

 homogeneous substance of uniform density. If the ever-increasing tension resulting from 

 the unequal storing of water reaches a point where the elasticity can no longer withstand 

 the tension, the substance of the center of the grain must expand and assume a condition 

 of greater swelling, and become less refractive. Observation shows, he states, that when 

 the grain has attained a certain size during swelling, the feebly refractive but much swollen 

 hilum appears at the center. The formation of the hilum is therefore brought about by 

 the action of tension, this tension being caused by unequal deposition of water molecules, 

 and not (as was assumed by Niigeli) by the deposition of starch molecules. The formation 

 of the hilum decreases the tension of the grain, but with the storing up of new substance in 

 the dense layer around the hilum the tension increases until it finally is sufficient to over- 

 come the elasticity. The result is that the dense layer is differentiated into three layers — 

 a soft middle layer and dense inner and outer layers. The dense outer layer behaves just 

 as the first layer, and when its tension reaches a certain intensity it also undergoes division 

 into a soft inner layer and a dense outer layer. As a result of the storing of new substance 

 in the grain the inner parts are expanded, the soft layers increase in density, and the dense 

 layers increase their water-content as they become softened, and, consequently the inner 

 parts of the grain are less resistant to swelling and solvent media than the outer parts. 



Schimper holds that the form of the starch-grain is determined by the manner of 

 "nourishing." Concentric grains result, he states, when they are completely surrounded 

 by the starch-producing substance or chloroplast, while eccentric grains are found when 

 the plastid is in contact with a part of the surface of the growing grain, the most rapid 

 growth taking place at the point or points of contact of the starch-forming substance. 

 Flat grains with a central hilum originate in lens-shaped chlorophyl-grains, and their flat 

 sides are parallel to the flat sides of the chlorophyl-grains. Elongated starch-grains are 

 formed in spindle-shaped chlorophyl-grains. The different shapes of the starch-grains can 

 be explained only by unequal "nourishment." If we try, he states, to conceive the manner 

 in which the grain is nourished by its mother substance, we imagine the latter to be a form 

 of solution which impregnates the chlorophyl-grain. The starch-grain and the chlorophyl- 

 grain which forms it do not lie in the cell-sap, but are embedded in the protoplasm. 



