CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 593 



(ri) Mechanical InJJuences. Organised structures bear without injury small mechanical 

 disturbances ; they are either sufficiently elastic, like starch-grains and cell-walls, again 

 to bring to equilibrium the changes which are thus caused in their internal tension and 

 external form ; or they are inelastic like protoplasm and chlorophyll-grains, and can then 

 equalise small passive changes of form in another way. But stronger disturbances cause 

 disruptions which cannot be again effaced. The molecular structure of the separated 

 portions may however still be perfectly retained, as is shown by fragments of starch- 

 grains and cell-walls. This is still more evident in motile protoplasm, where the sepa- 

 rated portions of the previously continuous substance behave like so many individuals, 

 and have the power of independent motion ; as, for example, separated portions of 

 Plasmodia, the detached halves of the rotating protoplasm in the root-hairs of Hydro- 

 charis when contracted by a solution of sugar, &c. In the same manner two or more 

 separated portions of protoplasm may unite into a whole, as in the formation of large 

 Plasmodia and of zygospores, the impregnation of oogonia, &c. The only purely 

 mechanical mode in which complete destruction of an organic structure can be accom- 

 plished is by crushing ; /. e. by complete disseverance of its molecules and their subse- 

 quent promiscuous intermixture. In this case a chemical change usually directly follows 

 the mechanical destruction of the molecular structure of the protoplasmic substance. 

 In some cell- walls the mere interruption of continuity by a cut causes striking changes 

 in the adjoining and the more distant parts ; thus, according to Nageli, cell-walls of 

 Schizomeris that have been cut through become shorter and thicker to a remarkable 

 extent. 



{e) Changes in the molecular arrangement of organised structures caused by injurious 

 influences determining their death are often accompanied by striking changes in their 

 power of diffusion. With respect to starch and cellulose but little is known in this 

 respect ; but the phenomena connected with protoplasm, including the nucleus, are very 

 remarkable'. Normal living protoplasm does not, for example, absorb any colouring 

 material from the surrounding solution ; but as soon as it has been killed by heat or by a 

 chemical re-agent, the dissolved colouring material not merely penetrates into it, but 

 accumulates in it to such an extent that the dead protoplasm appears of a much deeper 

 colour than the surrounding solution of the colouring substance. Starch and cellulose, 

 on the contrary, even in a fresh unchanged condition, absorb from a solution of iodine 

 a comparatively much larger quantity of iodine than of the solvent, and become of a 

 much deeper colour than the surrounding solution ; the colour is also different, usually 

 blue, while the surrounding solution is yellowish brown. The protoplasm which fills the 

 cells and has been killed in any manner, by frost, heat, or chemical agents, is more 

 permeable (whether cellulose is so also is not knowm) ; 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 molecular 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 molecules. 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 molecules ; and this must necessarily 

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



1 Niigeli, Pflanzenphysiologische Untersuchungen, vol. I, p. 3 et seq.-Wxxgo de Vries, Sur la 

 permt'abilite du protoplasm des betteraves,_Arch. Neeiland, vol. VI, iSyr. 



Q q 



