CONDITION OF AGGREGATION OF ORGANISED STRUCTURES. 591 



penetrate from without into every point of the interior, and to be ao-ain conveyed 

 outwards. 



This internal instability attains its highest degree in chlorophyll-grains and pro- 

 toplasm. In the former chemical processes take place wiih great energy and activity 

 under the influence of light, such as the formation of the green colouring matter and 

 of starch ; and when deprived of light other chemical changes at once ensue, which 

 terminate only with the complete destruction of the entire chlorophyll-grain. The 

 remarkable properties of protoplasm, w^hich we have already examined from different 

 sides in discussing the structure of the cell, attain their climax in its spontaneous 

 automatic power of motion, and in its capacity of assuming different forms and 

 changing both its shape and its internal state, and therefore of bringing into action 

 internal forces, even when corresponding impulses from without cannot be observed. 

 It is impossible to enter here in detail into the explanation of these remarkable facts; 

 but they will be understood, at least generally and to a certain extent, if it is borne 

 in mind that neither the chemical nor the molecular forces are ever in equilibrium in 

 the protoplasm ; that the most various elementary substances are present in it in the 

 most various comiMnations ; that fresh impulses to the disturbance of the internal 

 equilibrium are constantly given by the chemical action of the oxygen of the air ; 

 and that forces are continually being set free at the expense of the substance of the 

 protoplasm itself, which must lead to the most complex actions in a substance of 

 so complicated structure. Every impulse fiom without, even when in-sperceptible, 

 must call forth a complicated play of internal movements, of which we are able to 

 perceive onl)- the ultimate effect in an external change of form. 



The destruction of the molecular structure of organised bodies may take place in 

 many different ways, and affords an insight into many physiological processes. 



The most important forces by which the molecular condition of organic substances 

 is permanently altered are changes, in temperature, chemical reagents, and substances 

 which have a powerful attraction for water. But these agencies do not in general 

 cause destruction until they have exceeded a definite degree of intensity; while dif- 

 ferent changes of temperature and different states of concentration of the reagents 

 not unfrequently give rise to phenomena differing not only in degree but even in kind. 

 The effect of most external influences depends moreover to a great extent on the 

 chemical nature of the substance which forms the material and molecular framework of 

 an organised body. Cellulose ^ and starch may therefore be distinguished from crystal- 

 loids, chlorophyll-grains, and protoplasm, the former consisting mainly of carbo-hydrates 

 insoluble in water, the latter chiefly of albuminoids. 



(a) Temperature does not usually cause any striking or permanent change or destruc- 

 tion of organisation till it exceeds 50^, or sometimes even 60° C, and the substance 

 affected is completely saturated with water. Air-dry organised bodies can generally bear 

 much higher temperatures without injury. Thus, for example, dense starch-grains con- 

 taining but little water are not converted into paste below 65° C, while the more watery 

 grains undergo this change at 55° G. (Nageli), the capacity for absorbing water and in 

 consequence the volume then increasing enormously. Payen gives the increase m volume 

 of starch in water of 60° C. as 142 p. c, at 70° to 72^ C. as 1255 P- c, the starch 

 originally containing, according to Nageli, only from 40 to 70 p. c. water. Air-dry 



1 The cell-wall I suppo^.e here and in the sequel to be neither culiculariscd, lignified. nor 

 converted into mucilajje. 



