36 LECTURE III. 



it, so the double refraction of a cell-wall or of a starch-grain 

 is due to the differences of tension in the lamellae of which it 

 is composed. If from any cause the differences are increased 

 or diminished, the double refraction will correspondingly 

 vary ; thus the double refraction of a thick cell-wall is more 

 marked than that of a thin one, and that of one which con- 

 tains a small proportion of water than that of one which 

 contains a larger proportion. 



Strasburger's view is borne out in a striking manner by 

 the fact, first observed by von Mohl, that the interference 

 colours presented, by starch-grains when examined with a 

 polarising microscope are precisely complementary to those 

 presented by unaltered cell-walls ; where the one is coloured 

 yellow, for instance, the other is coloured blue, and vice versa. 

 The position of the colours in starch-grains corresponds to 

 that in a piece of glass which is under traction, the position of 

 the colours in the cell-wall to that in a piece of glass which is 

 under compression. The former arrangement of the colours 

 is said to be positive, the latter negative. Strasburger has 

 shewn that a starch-grain tends to increase in size, whereas a 

 cell-wall tends to contract : the tension of the one is of the 

 nature of traction, of the other compression. The inference is 

 that the different position of the interference colours is due, 

 as is the case with glass, to the difference of tension. The 

 correctness of this inference is confirmed by Strasburger's 

 observation that when a cell-wall is becoming cuticularised its 

 optical properties become reversed. Von Mohl had observed 

 that cuticularised cell-walls are, like starch-grains, optically 

 positive, but Strasburger was, I believe, the first to trace the 

 gradual change in the optical properties of the cell-wall which 

 accompanies its cuticularisation. This change is due, accord- 

 ing to Strasburger, to the fact that the lamellae which are 

 undergoing cuticularisation increase somewhat in volume, and 

 consequently the tensions existing in the unaltered cell-wall, 

 and with them its optical properties, become reversed. 



In discussing the molecular structure of organised bodies 

 we have confined our attention almost entirely to cell-walls 

 and starch-grains ; in concluding the subject we may briefly 



