STARCH-GRAINS. 



59 



grains would be found the outermost layer of which would be a watery one; this, 

 however, never occurs; the outermost layer is always the densest and least watery. 

 According to this supposition the nucleus would also possess the properties of the 

 youngest grains, whereas the nucleus is always soft, the youngest grains dense. The 

 theory of deposition could only explain the formation of partially compound grains 

 if we suppose that the common layers had been subsequently deposited round two or 

 more previously isolated grains ; but the com- 

 mon layers would have a different form, and 

 the fissures in the interior of such grains 

 remain unexplained. The theory of deposi- 

 tion, finall)', is incompetent to explain why, 

 in the secondary grains, the strongest growth 

 always takes place in the line joining their 

 nuclei (Fig. 49). The older hypothesis of a 

 deposition of new layers from within pre- 

 supposes that the starch-grains were at first 

 lollow vesicles, which has never been ob- 

 served; on this hypothesis, moreover, it cannot 

 )e explained how the phenomena arise which 

 )ccur in the formation of secondary grains; 

 md this hypothesis must moreover suppose 

 [growth by intussusception to explain the 

 superficial extension of the layers. The hy- 

 fpothesis of growth by intussusception affords 

 :the simplest explanation of all the pheno- 

 • mena; and, after Nageli's researches, may be 

 'Considered as a fully established fact. The 

 laterial which penetrates into the grain, and 

 there becomes deposited in the form of new 

 I particles of starch, is, of course, in solution ; 

 [but its chemical nature is not yet certainly 

 [known; dissolved starch can never be proved 

 'to exist in the plant, at least in those cells 

 where active formation and growth of starch- 

 ^grains has been observed. It is, however, 



probable that a solution of sugar contained in the protoplasm is the material out of 

 [Which particles of starch are formed by chemical and physical changes. The starch 

 is easily changed into sugar by different agencies. From various facts (e.g. the pro- 

 [duction of radial fissures on drying), it must be concluded that the molecules of starch 

 lave not only a definite order of deposition in the direction of the radii, but are also 

 [arranged tangenitially in a definite manner in each layer. A corresponding stratified 

 J^structure with radial striation, and the consequent formation of areolae, has, however, 

 been observed only occasionally and imperfectly. 



Growth by intussusception depends on the permeability of all parts of the grain to 

 swater and aqueous solutions. This can only be explained by supposing that the 

 ^substance of starch is not continuous, but consists of distinct molecules, each of which 

 [Possesses the power of attracting water, and surrounds itself with an aqueous enve- 

 ilope; the molecules of starch are separated from one another by these aqueous 

 [envelopes ; the smaller the molecules in a given portion of a starch-grain, the thicker 

 [are these envelopes and the more watery the particular portion. From this it results, 

 I on purely mechanical principles, that, as the molecules increase in size, the aqueous 

 ^envelopes become thinner, and the molecules approach nearer one another. The 

 eatery layers therefore consist of small molecules which are separated by thick 

 iqueous envelopes, the denser less watery layers of larger molecules with thinner 



(X 



Fig. 49.— Starch-g^rains from the tuber of a potato 

 00). A an older simple grain ; £ a partially compound 

 grain ; C, D perfectly compound grains ; // an older grain, 

 the nucleus of which has divided ; a a very young grain, 

 b an older grain ; c a still older grain with divided nucleus. 



