664 MOLECULAR FORCES IN THE PLANT. , 



These facts, in connection with a number of other phenomena, first led Nageli 

 to the hypothesis that organised bodies consist of isolated particles or MicellcE^ 

 between which the water penetrates, and which are solid and relatively unchange- 

 able, and invisible even with the most powerful microscopes. Every micella of a 

 saturated organised body is, on this hypothesis, surrounded by layers of water by 

 which the adjacent micellae are completely separated from one another. These 

 micellae may be supposed to be of various sizes, and it is evident a priori that, if 

 the thickness of the aqueous envelope is the same, larger micellae will form a 

 denser, smaller micellae a less dense substance ; and it may therefore be concluded 

 conversely that the layers and lamellae of organised bodies of different densities, 

 especially those of the cell-wall and of starch-grains, are composed of micellae of 

 different sizes ; and the difference in the proportion of water in such cases leads to 

 the hypothesis that the densest substance consists of micellae which are several 

 thousand times larger than those of the more watery substance. As the micellae 

 increase in size, the density of the whole substance is moreover increased by the 

 smaller distance that intervenes between them, so that larger micellae are separated 

 from one another by thinner layers of water. The changes in volume of organised 

 bodies due to the removal of water or its absorption depend, according to this view, 

 on the fact that when swelling takes place the micellae are forced further apart by 

 the water which penetrates between them ; while, on the other hand, when water is 

 removed they approach one another in proportion as the water is withdrawn from 

 their interstices. 



The forces which are concerned in these processes in the interior of an organ- 

 ised body may be divided into three kinds : — (i) the Cohesion within each separate 

 micella impermeable to water, which is itself an aggregate of molecules and atoms ; 

 (2) the Attraction of the adjacent micellae for one another, in consequence of which 

 they tend mutually to approach; and (3) the Attraction of the surfaces of the micella 

 for the absorbed water, which counteracts the mutual attraction of the neighbouring 

 micellae. 



In starch-grains, cell-walls, and to a certain extent in crystalloids^, the absorbed 

 water is not deposited uniformly in all directions ; the micellae are, on the contrary, 



^ [The term Micella was applied by Nageli to the aggregates of molecules of which organised 

 bodies consist in the second edition of his work on the Microscope (1877). Pfefifer, in his Osmotische 

 Untersuchungen, published in the same year, applies the general term tagma to all aggregates of 

 molecules, and the term syn'agma to bodies which are built up of tagmata. In his Theorie der 

 Gahrung (1879), Nageli gives the following definitions of the terms which he suggests for describing 

 the constitution of matter: — Atom, the ultimate particle of a chemical element : Molecule, an aggregate 

 of atoms, and hence the ultimate particle of a chemical compound: Pleoti, an aggregate of molecules, 

 which, like the molecule, can be neither increased nor diminished without a change in its chemical 

 nature ; examples of this are afforded by compound salts {e. g. alum), and by salts which contain 

 water of crystallisation {hydropleon) : Micella, like the pleon, an aggregate of molecules, but differing 

 from the pleon in that it consists of a much larger number of molecules, and in that increase or 

 decrease of size does not affect its chemical constitution; in this latter respect it behaves like a 

 crystal. The micellse may combine to form a Micellar Aggregate {Micellverband), and this may be 

 so large that it is readily visible ; the crystalloids are examples of large micellar aggregates. It will 

 be noted that the Atom, Molecule, and Plepn are chemical ideas, whereas the Micella and the 

 Micellar Aggregate are purely physical.] 



^ [See Book I. p. 49, on Crystalloids.] 



