CH. I] ELODEA. 13 



than in Spirogyra, because here there are no chlorophyll 

 bodies to obscure the view. There is not only a layer of 

 protoplasm lining the cellulose wall of the cell, but a 

 complicated system traversing the cell-sap and connecting 

 the nucleus with the rest of the protoplasmic cell-body. 



The most striking fact visible in the Tradescantia hair 

 is the circulation of protoplasm, which is perhaps the best 

 ocular proof that can be given of the " aliveness " of a 

 plant-cell. The circulation is rendered visible by the 

 granules in the protoplasm which flow steadily along the 

 living ropes of which it consists. 



The leaves of the river- weed Elodea are useful on 

 account of the visibility of the circulating protoplasm in 

 their cells. In Elodea the chloroplasts differ from those 

 of Spirogyra in being small round bodies instead of spiral 

 ribbons; it is these bodies which make the circulation 

 visible as they glide round the cells carried along in the 

 flowing protoplasm. 



The young pith of elder (Sambucus nigra) is included 

 as a good general example of the plant-cell, in which the 

 cell-wall, the protoplasm, nucleus and vacuole can all be 

 studied *. The cell from the cortex of Ranunculus (fig. 3) 

 illustrates the same points. 



1 As the pith becomes old the protoplasm dies and the cell-contents 

 are replaced by air. The pith is then dry, white and very light. 



