CIRCULATION OF SAP. 87 



erate power ; and, furthermore, this rotation is constantly 

 in one direction, and if checked and then set in motion 

 again it proceeds in its original course, just as certain 

 twining plants will turn only in one direction. But the 

 rotary motion of the fluids in the cell does not prevent 

 a portion from passing through the cell walls, and the 

 peculiar action is kept up in all so long as active growth 

 proceeds. 



Boucherie, in his investigations upon trees in France, 

 found that felled trees continued to imbibe moisture 

 through their exposed cells with considerable force, and 

 that a Poplar ninety-two feet high absorbed in six days 

 nearly sixty-six gallons of pyrolignite of iron. We all 

 know that cut stems of plants, it placed in water, will keep 

 fresh a much longer time than if the lower ends are not 

 immersed, or in some other manner supplied with liquids, 

 and this is mainly, but not wholly, due to the absorption 

 through the exposed cells. It is evident that heat and 

 light have a powerful influence in the flow of sap in 

 plants, by promoting transpiration and action in the 

 cells, but imbibition of liquids by the roots does not 

 necessarily cease with growth of the plant, or even loss 

 of foliage, for as liquids of less density than those within 

 them are presented to the roots, absorption must con- 

 tinue, although the movement may be slow when the 

 plant is less active than during the growing season. We 

 conclude that this must occur from the fact that trees, 

 shrubs and other plants, while apparently at rest, even in 

 cold climates, become gorged with liquids, and at a 

 season when there cannot be any considerable exhalation 

 from the leaves of evergreens, or the twigs and buds of 

 deciduous kinds, which would promote or cause continue^ 

 absorption of liquids by the roots ; still, it is well known 

 to every investigator that exhalation from the parts of 

 plants exposed to the air does not cease altogether, even 

 in the coldest weather, and the loss of this moisture 



