ADDRESS. 1 1 



special groups of cells, which we call organs, and whose proper duty is 

 now to take charge of the special processes which have been assigned to 

 them. In all this we have a true division of labour, — a division of 

 labour, however, by no means absolute ; for the processes which are 

 essential to the life of the cell must still continue common to all the cells 

 of the organism. No cell, however great may be the differentiation of 

 function in the organism, can dispense with its irritability, the one con- 

 stant and essential property of every living cell. There thus devolves on 

 each cell or group of cells some special work which contributes to the 

 well-being of all, and their combined labours secure the necessary con- 

 ditions of life for every cell in the community, and result in those com- 

 plex and wonderful phenomena which constitute the life of the higher 

 organisms. 



We have hitherto considered the cell only as a mass of active 

 nucleated protoplasm, either absolutely naked, or partially enclosed in a 

 protective case, which still permits free contact of the protoplasm with 

 the surrounding medium. In very many instances, however, the proto- 

 plasm becomes confined within resisting walls, which entirely shut it in 

 from all direct contact with the medium which surrounds it. With the 

 plant this is almost always so after the earliest stages of its life. Here 

 the protoplasm of the cells is endowed with the faculty of secreting over 

 its surface a firm, resisting membrane, composed of cellulose, a substance 

 destitute of nitrogen, thus totally different from the contained protoplasm, 

 and incapable of manifesting any of the phenomena of life. 



Within the walls of cellulose the protoplasm is now closely imprisoned, 

 but we are not on that account to suppose that it has lost its activity, or 

 has abandoned its work as a living being. Though it is now no longer 

 in direct contact with the surrounding medium, it is not the less dependent 

 on it, and the reaction between the imprisoned protoplasm and the outer 

 world is still permitted by the permeability of the surrounding wall of 

 cellulose. 



When the protoplasm thus becomes surrounded by a cellulose wall it 

 seldom retains the uniform arrangement of its parts which is often found 

 in the naked cells. Minute cavities or vacuoles make their appearance in 

 it ; these increase in size and run one into the other, and may finally form 

 one large cavity in the centre, which becomes filled with a watery fluid, 

 known as the Cell Sap. This condition of the cell was the first observed, 

 and it was it which suggested the often inapplicable term ' cell.' By the 

 formation of this central sap cavity the surrounding protoplasm is pushed 

 aside, and pressed against the cellulose wall, over which it now extends 

 as a continuous layer. The nucleus either continues near the centre, 

 enveloped by a layer of protoplasm, which is connected by radiating 

 bands of protoplasm with that of the walls, or it accompanies the dis- 

 placed protoplasm, and lies embedded in this on the walls of the cell. 



We have abundant evidence to show that the imprisoned protoplasm 

 loses none of its activity. The Gharacce constitute an exceedingly in- 



