212 PROTOPLASM. 



587. Up to the present time the microscope has not revealed 

 more than these facts respecting the intimate structure of proto- 

 plasm, and from these alone no clear conception can be formed 

 of the mechanics ' of protoplasmic movements. 



588. It is just at this stage of the inquirj- respecting the 

 structure of protoplasm that many have sought to apply an 

 hypothesis known as Ntigeli's ; namely-, that all organized 

 bodies consist of structural particles (termed micellce), each of 

 whieli is individually enveloped bj- a film of water holding vari- 

 ous substances in solution. According to Nageli's view, as origi- 

 nally given, tlie micellae are never spherical, but possess a true 

 crystalline character, as shown bj- the relations of organized 

 bodies to polarized light.^ These micellae are believed to obey 



1 Hofmeister regarded protoplasmic movements as directly dependent upou 

 changes in the capacity of living jirotoplasm for absorbing water, shown by 

 pulsating vacuoles (see 120). In the mass of a plasmodium, or in the free 

 spores of some algse, there are generally to be detected easily under the micro- 

 scope minute spherical cavities filled with watery sap which are constantly 

 changing in size. Tlieir rhythm of change, or pulsation, as it is called, is differ- 

 ent for different plants, varying from a few seconds to as many hours. Their 

 increase iu size is usually gradual until the maximum is reached, when sud- 

 denly the cavity or vacuole contracts even to the point of vanishing, and 

 then it slowly begins to form again at the same place iu the mass. The 

 rhythm of the pulsations can be made to vary with changes in the surround- 

 ings ; for instance, with changes of temperature, or by the application of dilute 

 solutions, or by any agent which modifies the absorptive power of proto- 

 plasm for water. But these agents are also efficient in controlling the rate 

 of protoplasmic movement. The spontaneously pulsating vacuoles appear to 

 indicate that the absoi-ptive power of protoplasm changes spontaneously, and 

 is different successively in different parts of the mass, thus disturbing the 

 equilibrium of the soft mass sufficiently to force some portions from place 

 to place. Bnt Hofmeister gave no explanation of the cause of variations in 

 the imbibition power of proto[ilasm. 



2 In his earliest work on the subject (Die Stiirkekorner, 1858) Nageli applied 

 the word molecule (which had not then obtained such general acceptance in 

 chemistry and physics, with a different signification) to what he now calls the 

 micella. His hypothesis has undergone sundry changes from time to time, 

 one of his last important publications (Theorie der Garung, 1879) containing 

 some modifications. 



The terminology now proposed by Nageli applies the word p^eon to those 

 aggregates of molecules which cannot be increased or diminished without 

 changing their chemical nature ; for instance, crystals which contain water of 

 crystallization would be called pleons, for the molecule HjO has a definite 

 numerical relation to the molecules of the salts, and examples of similar pleons 

 are afforded by such compound .salts as the alums. 



Compare with this the following statement : — 



" It has also been a question among chemists whether molecular combination 

 was possible ; in other words, whether it is possible for molecules of different 



If 



