THE CELL- WALL. 



483 



the conclusion that all organic substances are composed of crystalline 

 molecules grouped in a definite manner. When dry the molecules are 

 without interspaces ; when moist, each molecule is surrounded by a thin 

 film of water. Nageli further supposes that each molecule is made up of 

 a number of atoms, similar to or identical with the atoms of the chemist. 

 The molecules are of different sizes ; those portions of the structure richest 

 in water have the smallest molecules. The molecules themselves are of 

 the nature of crystals with two optic axes. It is possible that the ex- 

 tremely minute "dots and striae above mentioned may have some relations 

 to this molecular structure. 



Secondary Growths. The walls of almost all cells soon exhibit 

 a departure from the original simple condition, arising from the 

 formation of new lamellae, more or less resembling the primary 

 membrane, all over, or over particular parts of, the inside of the 

 primary membrane. These are distinguished as secondary layers 

 (figs. 529, 530). They are of different densities, and they are 

 usually separated one from the other by thin films of watery cell- 

 sap. The consistence of these layers, and the mode in which they 

 are disposed, produce the most important diversities of character of 

 the walls of fully developed cells. 



Fig. 530. 



Fig. 529. Transverse section of liber-cells of Cocos botryopkora. Magn. 2CO diam. 

 Fig. 530. Transverse section of a thick-walled cell from the pith of Roya carnosa. 

 Magn. 500 diam. 



The laminated condition of cell-membrane may be well observed in 

 simple cellular structures by treating fragments of Cladophora glomerata, 

 or other Large Confervoid, with diluted sulphuric acid. The laminae are 

 very visible in cross sections of the cells of wood and liber after these 

 have been boiled for a short time in nitric acid. These so-called layers 

 are not successive depositions, but are formed like the cell-wall itself, of 

 which, indeed, they are intrinsic portions, by intussusception of new 

 particles alternately more or less dense. The term layer is therefore a 

 misnomer, descriptive of an apparent and not of a real condition. 



2i2 



