73 o THE POPULAR SCIENCE MONTHLY. 



can dispense with its irritability, the one constant and essential prop- 

 erty 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 labors secure the necessary conditions of life for 

 every cell in the community, and result in those complex and wonder- 

 ful phenomena which constitute the life of the higher organisms. 



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

 ated protoplasm, either absolutely naked, or partially inclosed in a 

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

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

 toplasm 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 im- 

 prisoned, 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 pro- 

 toplasm 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 ap- 

 pearance in it ; these increase in size and run one into the other, and 

 may finally form one large cavity in the center, 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 inap- 

 plicable term " cell." By the formation of this central sap cavity the 

 surrounding protoplasm is pushed aside, and pressed against the cellu- 

 lose wall, over which it now extends as a continuous layer. The 

 nucleus either continues near the center, enveloped by a layer of pro- 

 toplasm, which is connected by radiating bands of protoplasm with 

 that of the walls, or it accompanies the displaced protoplasm, and lies 

 imbedded in this on the walls of the cell. 



We have abundant evidence to show that the imprisoned proto- 

 plasm loses none of its activity. The Characce constitute an exceed- 

 ingly interesting group of simple plants, common in the clear water of 

 ponds and of slowly running streams. The cells of which they are 

 built up are comparatively large, and, like almost all vegetable cells, 

 are each inclosed in a wall of cellulose. The cellulose is perfectly 

 transparent, and if the microscope, even with a low power, be brought 



