PROTOPLASM. 9 



on the other hand, greedily absorbs colouring matters, and 

 usually becomes much more intensely coloured than sur- 

 rounding structures. Owing to the varying composition of 

 actively living protoplasm, no reliable chemical test for it 

 is known, in fact, the tests usually given depend on the 

 presence of various substances which are assumed to be 

 constantly produced by the activity of living protoplasm. 

 It becomes blackened after having been in contact for some 

 time with a very dilute solution of nitrate of silver (even 

 one part in 1,000,000 parts of water); this test proves 

 the presence of living protoplasm, the reduction of the 

 silver- solution being due to the reaction of the albumen of 

 living cells, the failure of this reduction in dead cells is due 

 to a chemical change having taken place in the albumen. 

 Molybdic acid gives to dead protoplasm a deep blue colour. 

 Very little is known respecting the structure of living pro- 

 toplasm. It has recently been described as consisting of 

 very delicate, spirally twisted, hollow tubes, composed of a 

 colourless, unstainable, somewhat gelatinous substance, that 

 easily swells up in water. These threads are in turn spirally 

 coiled and form the walls of hollow cylinders ; the hollow 

 tubes and the cylinders formed by them are filled with the 

 so-called granular protoplasm, in which the streaming or 

 movements take place. The spiral threads and the cavities 

 they form are the portions of physiological value, and these 

 are not isolated in each cell of the organism, but continuous 

 from one cell to another throughout the entire structure. 



The relation of life to surroundings has to some extent 

 been already demonstrated, nevertheless other far-reaching 

 or universal dependencies on the nature of the environment 

 require attention. 



(a) Food is the only source of material by which a Hving 



