32 



THE ELEMENTARY STRUCTURE OF PLANTS. 



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the microscope, currents, rendered more visible by the contained 

 granules or solid atoms, are seen flowing around the cell, or around 

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

 The cause of this curious phenomenon and the object it subserves 

 ai-e unknoAvn; 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 

 1^ ;|| ^j 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 Avell, 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 lai'ge stinsins 

 hairs of Nettles, and the bristles on the ovary of 

 Circaea, show this circulation very well. In the 

 latter, instead of the separate and slender stream- 

 lets of Tradescantia, we perceive a broad and con- 

 tinuous stream flowing up on one side of the long 

 cell, around the summit, and do-wn 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 green 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 Eehgrass of fresh 

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

 four hundi'ed diameters. Through their nearly transparent walls, a 

 current of protoplasm, usually carrying with it some globular loose 

 grains of chlorophyll, 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 enou2rh to set the lai-ge nucleus, or a central mass 









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

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



