STARCH-SUBSTANCE, AND THE STRUCTURE, ETC., OF THE STARCH-GRAIN. ' 35 



radial direction. He found that when artificial swelling media act upon the starch-grain 

 slowly, the volume of the grain is increased and radial cracks are formed which indicate 

 that during this process more water is stored radially than tangentially. 



The existence of tensions Niigeli deduced from the theory of growth by intussuscep- 

 tion. Starch-grains in which radial cracks appear on drying do not lose the cracks on 

 moistening, but on the other hand the cracks become larger and hence more evident. The 

 lamellae of the grain in its natural state have a tendency to store more water tangentially, 

 and therefore grow chiefly in length and breadth, and this tendency is more pronounced 

 in the external layers, so that these in contrast with the inner lamellae have a positive ten- 

 sion. He states that if this were not the case, on moistening the dried grains the cracks 

 would become imperceptible. Tensions could not occur, he holds, in growth by apposition. 

 If we imagine a starch-grain to lie in the cell-protoplasm or in the cell-liquid, it is in con- 

 tact with water, and it has absorbed as much water as it can, because the micellae attract 

 as much water as their molecular force can hold. The particles of micellae are formed in 

 such a manner that there is an equilibrium between the particles of starch and water. The 

 tensions determine the separation of the soft center or hilum and the softer layers of the 

 grain, and this is brought about by the simultaneous growth by intussusception. 



Another fact that Nageli assumes aids in disproving the apposition theory is the 

 presence of a thin peripheral layer which possesses different properties from the rest of 

 the grain. He states that when this outer layer is very thin it does not react with iodine, 

 and that when somewhat thicker it is colored a reddish-violet, while the part inclosed by 

 this layer always becomes a deep blue. This peripheral layer likewise resists the action 

 of solvents (acids, etc.), while the entire inner mass is dissolved. Since both large and 

 small grains have this peripheral layer or "membrane," growth by intussusception, 

 according to Nageli, is the only possible explanation of the mechanism of formation of 

 the starch-grain. 



The hypothesis proposed by Schimper that the starch-grain is a spherocrystal of a 

 carbohydrate was supported by Meyer (Botanische Zeit., 1881, xxxix, 841), who states 

 that it furnishes the simplest explanation for the lamellation, the apposition-growth of 

 these lamellae, the later deposition of the outer layers by external accretion, and the rela- 

 ti\'ely low densitj^ of the inner parts of the older starch-grains. He compares the growth, 

 the formation of lamellae, and the variations in density of the different parts of the grain 

 with similar phenomena of spherocrystals generally, and shows analogies in causes and 

 explanations. He reasons that if starch-grains are spherocrystals, it may be assumed with 

 certainty that they grow in a manner analogous to that of other spherocrystals. If sphero- 

 crystals of a carbohydrate, such as sugar, are caused to form, and if conditions attending 

 crystallization are altered periodically, as when crystallization takes place at a window 

 where the sun warms the preparation periodically, spherocrystals are deposited in the 

 form of a concentric lamellated structure; but if crystallization occurs under constant 

 conditions of temperature, the crystals show no lamellae. It is therefore to be inferred, 

 Meyer holds, that the formation of the layers of the starch-grain is due to fluctuations in 

 external conditions. The hila of the spherocrystals of sugar are usually less dense than 

 other parts of the crystals, and they inclose a mother substance, rarely air. Very seldom 

 is a dense liilum observed. 



Applying these facts to the growth of the starch spherocrystal, Meyer states that 

 they show that most starch-grains, even in young stages, must have a small, relatively 

 soft center; and that the starch-grains, since the assimilation of the plants is subject to 

 periodic fluctuations, must be built up of layers of alternating density. The youngest, 

 outermost layer is always the most dense, and the successive, deeper-lying layers become 

 less dense according to their age. The varying density, he assumes, is due to the action 

 of ferments. Thus, if one assumes that a layer just formed is exposed to ferment action 



