CHAP, iv.] and of growth by intussusception. 353 



differentiation in three directions in space of the substance of 

 every minute portion of cell-membrane, and made better use 

 than von Mohl himself had made of the comparison which he 

 had suggested, namely, that the structure of a cell-wall with 

 cross-striation and at the same time with concentric stratification 

 resembles that of a crystal cleaving in three directions. He 

 first gave expression to this conception of the structure of the 

 cell- wall in 1862 in his ' Botanische Untersuchungen,' I. p. 187, 

 and further developed it in the second volume of the same 

 work at p. 147. 



But the true starting-point of Nageli's theory of molecular 

 structure is to be found in his searching investigations in 1858, 

 into the constitution of starch-grains. From the way in which 

 they resist the effects of pressure, drying, distention, and with- 

 drawal of a part of their substance, he arrived at the conclusion 

 that the whole substance of a starch-grain is composed of 

 molecules, whose shape must be not spherical but polyhedral, 

 that these are separated from one another in their normal 

 condition by envelopes of water, and that the amount of water 

 in the stratified substance depends on the size of these 

 molecules, the water being less when the molecules are larger ; 

 this view could at once be applied to the structure of the cell- 

 wall, the growth of which may be explained as the increase in 

 size of the molecules already present, and the intercalation of 

 new small molecules between the old ones. These molecules 

 of Nageli are themselves very compound bodies, for the smallest 

 of them would consist of numerous atoms of carbon, hydrogen 

 and oxygen, and ordinarily a molecule would be composed of 

 thousands of those aggregates of atoms, which the chemists 

 call molecules. 



In examining starch-grains Nageli came to the conclusion 

 that molecules of different chemical character are grouped 

 together at every visible point ; the material which colours 

 blue with iodine, the granulose, could be removed from the 

 grains, and then there remained behind a skeleton of the 



A a 



