670 MOLECULAR FORCES IN THE PLANT, 



permeable (whether cellulose is so also is not known) ; it allows the cell-sap, which in 

 living and growing cells is always subject to high pressure, to filter out as if it had 

 become porous. This is well seen when coloured cells or tissue are frozen or heated 

 above 50° C. ; they then allow their coloured contents to diffuse out, which they do 

 not do when living. 



(/) The true nature of the change which the micellar structure of moist organ- 

 ised bodies undergoes by heating above 50° or 60° C, or when they are made to swell 

 up strongly by treatment with acids or alkalies, is considered by Nageli to lie in the 

 destruction of the crystalline micellae. In the case of starch-grains and cell-walls this 

 view is supported by a few facts which have hitherto not been explained in any other 

 manner. The increase of the power of absorbing water under such conditions is then 

 explained on the hypothesis that the number of particles which attract water is increased 

 and their size diminished by the destruction of the micellae ; and this must necessarily 

 be connected with an increase in the proportion of water and a corresponding increase 

 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 micellae, the explanation may lie in the fact that the large micellae 

 of the dense substance are broken up into a number of small micellae, 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 micellae 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 micellar 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 water they are hard, then tenacious, then viscous and 

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

 the fluid state they may be mucilaginous, adhering strongly to organised, less strongly to 

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

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

 they afford a homogeneous substance which differs greatly in its capacity for swelling and 

 in its optical properties from crystals and from organised bodies. In contradistinction to 

 these latter, colloidal bodies may be considered amorphous internally as well 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 



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



