3l6 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



serrated line in Fig. 1 5 6. The arrangement, therefore, of the layers is 

 not dense/light/dense, but rather a gradual reduction in density in 

 each layer up to the edge of the next layer, at which point the density 

 suddenly rises again to its initial level. The fact noted by Young 

 (1938) that the layers of the starch grains of Canna are more easily 

 stained outside than inside is probably due to the looser structure of 

 the outer portions of the layers. 



The inference from the foregoing as to the apposition growth of 

 starch grains is that, in the formation of a new layer, the deposition 

 is at first dense, becoming looser little by little until, at a given degree 

 of impoverishment, growth ceases altogether. The sugars consumed 

 have then probably to be made good before the process can start 

 afresh. As Van de Sande Bakhuizen (1925) showed, if external con- 

 ditions are constant, lamination does not occur, because nutritive 

 material is then always available in the same concentration and, 

 therefore, there is no impoverishment during growth. The same 

 applies to the lamination of cotton fibres, which likewise depends 

 largely upon external conditions of growth, viz., temperature (Kerr, 

 1937), or can, indeed, be prevented altogether by constant exposure 

 to light and the exclusion of fluctuations in temperature (Anderson 

 and Moore, 1937). During their entire growth, the starch grains are 

 enclosed within the amyloplast, which produces them; this stretches 

 very considerably in the process and finally becomes an exceedingly 

 thin, scarcely perceptible pellicle enveloping the grain. 



Radial structures have for long been observed in addition to lami- 

 nation ; they take the form of corroded patterns during the mobili- 

 zation of the starch in the germinating seeds, or of thin radial cracks. 

 The starch grains have therefore been thought to be of spherite tex- 

 ture. This would seem to receive support from the optical fact that 

 a positive spherite cross always occurs (see Fig. 66, p. 96), because 

 the starch chains, like cellulose chains, are optically positive as referred 

 to their long axis. 



The starch grains can be split up tangentially and radially by chemi- 

 cal means into minute blocks of i ^ edge length (Hanson and Katz, 

 1934; Badenhuizen, 1937) and these particles have been said to be 

 pre-formed elementary units of the starch grain (Fig. 157). Structures 

 of the kind are obtained if starch granules are treated for days with 

 7^4% hydrochloric acid and are then swollen in 2 molar Ca(N03)2 



