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182 LECTURE X. 



watery : from this he argues that the layers cannot be successively depo- 

 sited for, were that the case, the most external layer would be the youngest 

 and therefore probably the most watery part of the grain. Schimper, 

 however, has pointed out that, under certain circumstances, a starch- 

 grain which has lost its regular outline in consequence of partial solution, 

 may have new layers deposited upon it with a regular outline, and that, 

 the irregular outline of the corroded grain can still be seen within; 

 this fact affords considerable support to the apposition theory. The 

 stratification of starch-grains, he admits, is due, as Naegeli states, to 

 the alternation of more and less watery layers ; he ascribes this distribu- 

 tion of water to tensions in the grain which cause each apposed layer to 

 become differentiated into three, a middle watery layer with a dense layer 

 on each side of it. Strasburger denies the alternation of more and less 

 watery layers. He comes to the conclusion that, in starch-grains as in cell- 

 walls, the layer last formed (i.e. the one next the cell-contents) is the most 

 dense one, and that the older layers gradually absorb water : hence the 

 external layer of a starch-grain is the most dense, and the inner layers are 

 successively more and more watery until the maximum proportion of 

 water is reached in the hilum. Each layer is therefore stretched by the 



FIG. 27. Starch-grains (Potato) under the polariscope: A, (after Dippel) a starch- 

 grain seen with crossed Nicols: B (after Weiss) a starch-grain seen with 

 parallel Nicols. 



layer within it (positive tension), compressed by the layer external to it 

 (negative tension). The tensions in a starch-grain are thus precisely the 

 opposite of those in a thickened cell-wall (see p. 36), but this statement 

 of them applies quite accurately only to concentric starch-grains : it is 

 true of the older concentric part of excentric" grains, and in these grains 

 the external layer is always the most dense, but the excentric incomplete 

 layers are not successively more and more watery from within outwards. 



