134 MOVEMENTS IN CELLS. - ROTATION. 



easily seen. In Chara, also, he states it is not the cell-sap which moves, 

 but a denser fluid, present in large quantity, and occupying the outer 

 parts of the cell-cavity. Mold thinks that a homogeneous protoplasm 

 fills these cells at first completely, but that during growth it becomes 

 hollowed out into one or more cavities, and that around these the 

 mucilaginous matter circulates. In Vallisneria, there is only one 

 cavity, while in other plants there are several, giving rise to the 

 appearance of mucilaginous streams or lines running from the nucleus 

 to the cell-wall These mucilaginous lines, he says, occasionally after 

 the circulation has ceased, remain permanently on the cell-walL The 

 existence of spiral fibres in cells has been traced to currents of this 

 kind. 



280. The velocity of the currents in various plants, at 66 to 68 

 Fahrenheit, is thus given by Mohl: 



Filamental hairs of Tradescantia virginica, ^ to ^j of a Parisian line in a 



second; mean, -5^. 

 Leaves of Vallisneria spiralis quickest, -5-^-5 ; slowest, -^^ ; mean, -j-Lg- of a 



line in a second. 



Stinging hairs of Urtica baccifera quickest, g^ T ; slowest, -g^ ; mean, yi^. 

 Cellular tissue of young shoot of Sagittaria sagittifolia, j^, to 1 ^ 56 ; mean, -g^. 



leaf of do., y^, to -j-jVtj ; mean, 

 Hairs of Cucurbita Pepo quickest, ^4^; slowest, 



The measurements were made by noting the passage of the globules 

 across the field of a micrometer, fixed in the ocular of the microscope, 

 and counting the strokes of a second's pendulum. These movements 

 appear more rapid to the observer; but then it must be recollected 

 that the parts are seen in a highly magnified state. 



281. The Cause of Rotation has not been satisfactorily explained. 

 Some attribute it to electrical or magnetic currents causing attrac- 

 tion and repulsion of the granular contents of cells. The different 

 contents of the cells, according to them, mutually act and react on 

 each other, and thus give rise to movements similar to those which 

 take place on the surface of water when oily or resinous matters are 

 added, and which have been called epipolic (liriK&qs, on the surface). 

 Recent observations, by Dutrochet, seem to show that the magnetic force 

 exercises no influence over the movements in Chara. Others believe 

 that while heat, and electricity, and physical agents, stimulate these 

 movements, they are not the cause of them. Some trace the move- 

 ments to the presence of the nucleus, and look upon them as connected 

 with the period of growth when new cells are being formed, and 

 as ceasing after the nucleus has disappeared. 



3 RESPIRATION OF PLANTS. 



282. The changes which are produced in the atmosphere by living 

 plants have been included under the title of Vegetable Respiration. 



