Protoplasm. 37 



morphological non -differentiation. Every part of the cell 

 is capable of taking in food-particles, the excreta being also 

 ejected at any point. Emission of carbonic acid or respira- 

 tion takes place over the surface generally, and so on. 



By far the majority of plants and animals are, however, 

 made of vast multitudes or collections of cells, some of 

 which are specially differentiated to perform one function, 

 whilst others are differentiated to perform another. All the 

 cells have the general characters we have already described 

 as belonging to protoplasmic units, but while their other 

 functions are in abeyance, some one function it may be the 

 power of contracting or of exhibiting responses to stimuli 

 is very highly developed. Collections of such similar 

 cells are said to be specialised for the performance of 

 one function. Naturally, also, their form and structure 

 becomes correspondingly modified. Such cell aggregates are 

 termed tissues. Thus we have muscular tissue, whose 

 special function is that of contraction ; nerve tissue, whose 

 special function is that of showing irritability in response to 

 stimuli or automatically ; connective tissue, where the cells 

 are modified to act as padding or as connecting links 

 between other tissues, and so on. Moreover, it is con- 

 ceivable that different parts of the plant or animal may con- 

 tain more than one kind of tissue. Such parts are termed 

 organs ; hence the term organic as applied to chemical com- 

 pounds found in the cells and tissues composing such organs, 

 organised as applied to the plant or animal possessing such 

 differentiation of parts, and organism used as a synonym for 

 living thing. 1 



A reference was made (at p. 35) to the power which cells 

 possessed of separating a part of themselves for the purpose of 

 reproduction and multiplication. When the organism con- 

 sists of a single cell, separation of, so far as we know, any 



1 It must be borne in mind, however, that these terms are applied 

 to many plants and animals which cannot be said to possess tissues, 

 much less organs. 



