234 PEOF. ATTFIELD — PHYSICS AND CHEMISIET 



the rise and general movement of sap in plants — a movement which 

 extends from the lowest rootlet to the topmost leaf or twig ? 



The movement of fluid in plants has been set down to atmo- 

 spheric pressure, to capillary attraction, to endosmotic action, and 

 to the indirect influence of wind and warmth on elastic tissues. 



Atmospheric pressure, however, will only sustain a column of 

 water to a height of about 34 feet, as seen in the case of a common 

 pump, whereas the height of my birch is 39 feet, and the top of a 

 tree is often scores and sometimes hundreds of feet from the ground. 



By capillary action water ascends in wetted tubes as narrow as 

 hairs {capillus, a hair) when they dip into water ; and the height 

 attained above the level of the water outside the tube, other things 

 beiug equal, is inversely proportional to the diameter of the tubes. 

 Thus, at summer temperatures, in a wetted tube one twenty-fifth 

 of an inch in diameter, water will rise about an inch and a quarter. 

 Such action is apparently insufiicient to account for the rise of sap 

 in trees. 



With regard to endosmose, Dutrochet found that a tied bladder 

 containing a saline solution decreased in weight when placed in 

 water, and that a tied bladder containing water increased in weight 

 when placed in a saline solution. The saline solution passed 

 through the wall of the bladder or cell in one direction quicker 

 than water passed through it in the other direction. This action 

 between a weaker and a stronger fluid he called endosmose. Each cell 

 of a plant is the analogue of the tied bladder, and between one cell 

 and another there will be the passage of fluid whenever the densi- 

 ties of the two fluids vaiy. This action proceeds from cell to cell 

 throughout a plant, and hence may account for slow movements of 

 fluids within plants. But the tips of roots do not necessarily dip 

 into such an amount of water as would seem to be necessary were 

 endosmose the prime cause of the ascent of sap. I could not dis- 

 cover after much careful search that the rootlets of my birch were 

 in contact with actual fluid water. They seemed rather to termi- 

 nate in minute moisture -laden air spaces. Besides, as I understand 

 endosmose, its very existence depends on concurrent exosmose. 

 Hence the inflow of, say, common water to a plant containing its 

 minute proportions of saline matters should be accompanied by an 

 outflow of a fluid of different density, either absolutely pure water 

 on the one hand, or, on the other, sap containing elaborated material 

 such as sugar. I am not aware, however, that the outflow of either 

 kind of fluid has ever been observed, or that any observer contends 

 that it takes place. I cannot, at present, accept endosmose as a 

 satisfactory and complete explanation of the outflow from the 

 branch of my birch. 



Herbert Spencer says that wind in bending twigs, branches, and 

 trunks gives alternate squeezings to one side and the other and 

 corresponding extensions of tissue on the opposite sides, and that 

 this action sets up currents of sap within the tissue. I have great 

 respect for this eminent and far-seeing sociologist, and I doubt not 

 that wind exerts important actions upon plants, but my birch was 



