THE GROWTH OF STARCH-GRAINS 35 



stem, as can easily be seen after the cell has been dyed, and has subsequently 

 been passively stretched to a certain extent l . More frequently a plastic 

 stretching of the walls of living cells (collenchyma, &c.) is produced as the 

 result of external tension. Unequal strains may even be produced in the same 

 cell-wall by the dissimilar rates of growth of its different layers, and it is in this 

 way that the cuticle may be separated and thrown off. 



None of the above facts are contradictory to the supposition that the protoplast 

 may exert a controlling influence upon the growth of the outermost layers of 

 the cell-wall, or even upon the contiguous walls of a dead cell. This last 

 may in fact occur during the stretching of the walls of a dead spiral tracheide, 

 perhaps by the aid of the softening action exercised by excreted enzymes. It is, 

 indeed, difficult to see why the influence of the protoplast, which undoubtedly 

 extends to the middle lamella, might not reach as far as the directly continuous 

 wall of a neighbouring cell. 



SECTION 10. The Growth of Starch- grains. 



Starch-grains, whether formed by chloro- or leucoplastids, are able 

 in virtue of their power of imbibition and swelling to take up dissolved 

 substances, and hence to interpolate new particles between the older ones. 

 They might therefore grow by intussusception, although as a matter of fact 

 the researches of Schimper and of Meyer 2 have shown that starch-grains 

 usually grow by apposition. Whether an increase in size by intus- 

 susception may also occur is doubtful, for Nageli's arguments do not 

 suffice to establish this view 3 . 



According to Meyer, a starch-grain closely resembles a sphaerocrystal 

 in its mode of growth, and the resemblance holds good whether the 

 crystalline and anisotropous component particles are termed trichites 

 (minute crystals) or micellae 4 . The structure and lamellation of the starch- 

 grain are mainly the result of its growth by the apposition of successive 

 layers, but, as in the cell-wall, secondary modification is possible by means 

 of solvent and other agencies acting on the substance of the starch-grain. 

 Meyer suggests that the watery central layers result from a partial solution 

 of the starch first deposited, but it is also possible that the later layers 

 may be denser at the outset. Starch, like reserve cellulose, may be 

 partially or entirely dissolved when it is required as food, and hence at 



1 Nathansohn, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 671. The oider literature is given by 

 Nathansohn. 



a A. Schimper, Bot. Zeitung, 1881, p. 185 ; A. Meyer, Unters. iiber d. Starkekbrner, 1895. 



3 Nageli, Die Starkekorner, 1858, p. 213. Cf. II, p. 33. Nageli's arguments are given by Meyer, 

 1. c., p. 138. Cf. also Nageli, Bot. Zeitung, 1881, p. 633. 



4 Cf. also Biitschli, Unters. iiber Structuren, 1^98, p. 300. 



D 2 



