94 THE CELL 



little or no differentiation between different cells, but 

 in the higher plants it is considerable. 



A set of similar cells which perform one special 

 function or set of functions in the organism is called a 

 tissue. 1 Thus in the higher plants there are absorbing 

 tissues, conducting tissues, sugar-forming tissues, pro- 

 tective tissues, etc. Specialisation of different cells for 

 different functions reaches a far higher pitch in the 

 higher animals, where different tissues are associated 

 in the different organs, and work together to perform 

 the special function of the organ, e.g. lungs, heart, 

 liver, kidneys, etc. 



However specialised the structure and work of a 

 cell may be, it must necessarily be able to carry out 

 the vital functions described in the last chapter, or it 

 would not be alive. It depends, however, upon its 

 position in the body and its relation to other cells and 

 tissues for the essentials of its continued existence- 

 it cannot lead a free independent life like the cell of 

 a Protozoon and Obtain its food and oxygen directly 

 from outside the organism. Thus the muscle cells 

 depend for their food and oxygen on the circulation 

 of the blood which carries these necessary substances 

 to them. ; Each highly differentiated cell or tissue 

 specialises on one function of protoplasm, which repre- 

 sents its particular work for the organism as a whole. 

 Thus the fibres of the muscle tissue (Fig. 5, F) show 

 the power of contracting a specialised form of move- 

 ment in a very high degree, and by means of the 

 co-ordination of the contractions of many muscles the 

 organism moves. The cells of the glands attached to the 

 alimentary canal specialise in the production of enzymes 

 which they pour out into the cavity of the gut for 

 1 From the analogy of its texture with that of a textile fabric. 



