356 Nageli's theory of molecular structure. 



of them which bore on the molecular structure of cell-walls 

 and starch-grains (' Botanische Mittheilungen,' 1862). The 

 phenomena of polarisation led him once more and by a 

 different path to the view that the organised parts of the 

 vegetable cell consist of isolated molecules surrounded by 

 a fluid, and his renewed investigations of these phenomena 

 resulted in more definite conceptions of the nature of these 

 molecules, which from the optical behaviour of the objects 

 examined he concluded were not only polyhedral but crystal- 

 line ; in effect, the molecules of the substance of the organised 

 parts of plants behave, according to Nageli, as crystals with 

 two optic axes, which therefore possess three different axes 

 of elasticity ; in starch-grains and cell-membranes these 

 crystalline molecules are so arranged that one of these axes 

 is always perpendicular to the stratification, while the two 

 others lie in its plane. The effect of the organised parts of the 

 cells on polarised light is the sum of the effects of the single 

 molecules, whereas the fluid that lies between them is optically 

 inactive, and only comes into consideration because according 

 to its quantity the molecules separate more or less far from or 

 approach one another. 



