^y4 MOLECULAR FORCES TN THE PLANT. 



in volume. It is especially noteworthy that the denser layers of starch-grains and cell- 

 walls become under these circumstances homogeneous with the least dense and most 

 watery layers. But since the denser layers probably consist of large, the less dense 

 layers of small molecules, the explanation may lie in the fact that the large molecules 

 of the dense substance are broken up into a number of small molecules, and thus 

 become similar to those of the less dense substance. The same explanation may be 

 given of the fact that when the organised structure is changed by undergoing strong 

 swelling, the optical properties of starch and cellulose also undergo change ; their pre- 

 vious action on polarised light disappearing altogether. This is also explained if we 

 suppose that under the action of these agents the molecules which produce the optical 

 effect lose their form, and that their fragments are irregularly intermixed. 



How far these views can be applied also to protoplasmic structures and their coagu- 

 lation remains at present uncertain. 



(g) The disorganisation of the molecular structure of organised bodies may take place 

 gradually ; and when it has exceeded a certain limit, a new substance is produced from 

 the originally organised material, the molecular condition of which has, since the time 

 of Graham, been termed colloidal. From the similarity which, according to Nageli and 

 Schwendener, exists between organised and crystalline bodies, it is not surprising that 

 there are also mineral substances, which, like silica, are usually crystalline, but become 

 under certain circumstances colloidal ^ Organised bodies absorb water and other fluids, 

 increasing at the same time in volume up to a certain maximum at which they are 

 saturated ; crystalline bodies dissolve in a definite minimum of water and produce a 

 saturated solution which can be diluted ad libitum. Colloidal bodies show in this respect 

 intermediate properties ; they can be mixed with water in all proportions without any 

 minimum or maximum. Solvents cause in organised and crystalline bodies a sudden 

 passage from the solid to the fluid condition. Colloidal bodies pass from the solid to 

 the fluid condition, when they are soluble, through all stages of softening ; in a certain 

 state when they contain but little w^ater they are hard, then tenacious, then tough and 

 scarcely fluid, finally when mixed with abundance of water perfectly fluid. Even in 

 the fluid state they may be mucilaginous, cohering strongly to organic, less strongly to 

 crystalline substances ; and even when greatly diluted they diflTuse very slowly, and some 

 of them appear unable to penetrate organic membranes such as cell-walls. On drying 

 they afl^'ord a homogeneous substance which diff'ers greatly in its capacity for swelling and 

 in its optical properties from the molecular structure of crystals and of organised bodies. 

 In contradistinction to these latter, colloidal bodies may be considered amorphous in- 

 ternally as wefl as externally. Colloidal bodies occur abundantly in plants as products 

 of the decomposition of organised bodies, and under certain circumstances they supply 

 material for the production of new organised bodies. Thus gum-bassorin and perhaps 

 also gum-arabic, as well as the mucilage of quince and linseed, result from the decom- 

 position of cell-walls ; perhaps also the formation of the substance of the cuticle must 

 be included in this category. Viscin is the product of decomposed cellulose ; the origin 

 of colloidal pectin and caoutchouc is still unknown ; but none of these substances are 

 of any further use to the plant. 



(/j) Traubes Artificial Cells-. Among the most important of the phenomena belonging 

 to the growth of the plant are those connected with the cell-wall ; and everything 

 which contributes to a more exact knowledge of its development must always be 

 welcome. The researches of Traube, of which an abstract is here given, are of great 

 interest from this point of view ; even though it may not always be possible to transfer 

 all the properties of his artificial cells to the real plant. 



^ See, among other authorities, Graham, Phil. Trans. 1862; Journ. Chem. Soc. 1862. 

 2 Traube, Experimente zur Theorie der Zellbildung u. Endosmose, in Arch, fiir Anat., Phys., 

 u. wissenscli. Medecin, von Reichert u. Dii Bois 1S67, p. 87 et sej. 



