THE ELEMENTAET STRUCTURE OF PLANTS. 





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the microscope, currents, rendered more visible by the contained 

 granules or solid atoms, are seen flo\Ying around the cell, or around 

 some portion of its periphery, in a circuit which returns upon itself! 

 The cause of this curious phenomenon and the object it subserves 

 ai-e unknown ; but it is doubtless a vital circulation, and not a 

 mechanical movement. In most plants it is not to be seen in 

 mature cells. But it may be observed in many 

 water-plants when full-grown, and in the hairs on 

 the surface of a great variety of land-plants. The 

 string of bead-like cells which compose the jointed 

 hairs of the common Spider Lily (Tradescantia, 

 Fig. 6) show this circulation well, under a magni- 

 fying power of about four hundred diameters. 

 With this power, a set of thread-like currents 

 may be seen to move between the cell-wall and 

 the enclosed colored contents, traversing the cell 

 in various directions, without much regularity, ex- 

 cept that the streamlets appear to radiate from, 

 and return to, the nucleus. The large stinging 

 hairs of Nettles, and the bristles on the ovary of 

 Circasa, show this circulation very well. In the 

 latter, instead of the separate and slender stream- 

 lets of Tradescantia, we jjerceive a broad and con- 

 tinuous stream flowing up on one side of the long 

 cell, around the summit, and down the opposite 

 side. This circulation may be more readily ob- 

 served in the cells of many aquatic plants. In 

 Chara and Nitella, — plants composed of large 

 cells lined with gixen granules, — a magnifying power of fifty or 

 one hundred diameters shows the circulation very well. And the 

 leaves of Vallisneria spiralis (the Tape-grass or Eel-grass of fresh 

 water) are still more beautiful objects, when magnified from two to 

 four hundred diameters. Through their nearly transparent walls, a 

 current of protoplasm, usually carrying with it some globular loose 

 grains of clilorophyll, may be seen coursing up the entire breadth 

 of the wall of each cell, across its summit, down the opposite side, 

 and across the other end to complete the circuit ; and often the 

 current is strong enough to set the lai-ge nucleus, or a central mass 



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FIG. 26. A few cells of the leaf of Naias (lexilis, highly magnified, showing the circulation ; 

 the direction of the currents indicated by arrow-heads. (Drawn by II. J. Clark ) 



