502 



PHYSIOLOGY. 



Starch-granules are commonly unaffected by cold water; but 

 when crushed, the inner layers will sometimes absorb it and swell 

 up. Boiling water causes them to swell up into a jelly, losing all 

 trace of their laminated structure as do also diluted sulphuric acid 

 and solution of potash. 



Iodine colours starch-granules violet, indigo-blue, or deep 

 blackish blue, in proportion to the degree of concentration in 

 which it is employed. By means of dilute sulphuric acid, starch 

 may be converted into dextrine and glucose. Modern researches 

 have shown that starch consists of two substances intimately com- 

 bined, one of which, granulose, is more soluble in saliva than the 

 other, cellulose ; and the action of iodine is also different in the 

 two cases. 



Mode of formation. Great discussion has taken place of late years as to 

 the structure and the mode of development of starch-granules. They are 

 apparently formed of a numher of concentric laminae, which increase in 

 density from within outwards. Their substance is hardly distinguishable 

 from that condition of cellulose where the cell-membrane swells into a 

 gelatinous substance with dilute sulphuric acid, or even sometimes with 

 water, and takes a more or less decided blue colour with iodine alone. 

 With regard to their mode of development, they appear to be formed 

 either by intussusception, as maintained by Na'geli and Sachs, or by the 

 deposition of successive layers of starch-substance, by protoplasm, in the 

 interior of vacuolar cavities formed in the protoplasmic matter of the cell, 

 either while this exists as a colourless mucilaginous matter, or after it 

 has become more highly organized into chlorophyll-corpuscles. Starch- 

 granules, in fact, appear according to this view, to be formed \)\ secretion 

 on the inside of a utricle of protoplasm, exactly in the same way as the 

 cellulose wall of the cell is secreted on the outside of the primordial 

 utricle. 



Fig. 552. 



Fig. 553. 



Fig. 552. Part of a cell of the stem of the White Lily: n, nucleus, surrounded by protoplasm 

 in which starch -granules () are being developed. Magn. 400 diam. 



Fig. 553. Starch of Maize: a, section of a young cell of the seed, with nascent starch -granules 

 imbi'dded in protoplasm ; b, section of a full-grown cell with the starch-granules 

 in contact and become angular by mutual pressure. Magn. 200 diam. 



This mode of development is well illustrated in the formation of starch- 

 granules in the cords of protoplasm which have ceased to circulate, in 



