OF THE SAP OF PLANTS. 235 



yielding; iioarly a gallon of sap daily when no wind was blowing, 

 and the anionnt was not pciroptibly altered when the wind did blow. 



The suggestion has been made that the warmth of spring expands 

 the solid parts of a plant, and that natnre abhorring a ^'acuum drives 

 in water to snpply the thus enlarged space or spaces. How any 

 such warmth expands the solid without at the same time expanding 

 the li(iuid and gaseous contents of a plant is not stated. Moreover 

 this driving up of water or sap is only a fresh name for the pressure 

 caused by the weight of the atmosphere, which pressure has already 

 been shown to be insufficient to account for all the facts of the case. 



" Root-pressure " is also a name wliich frequently occurs in the 

 vocabulary of some writers. And a harmless name it is for de- 

 scribing some of the effects we are considering. But regarded as a 

 cause I find it is only a fresh name for either atmospheric pressure 

 or for endosmose. 



Transpiration, that is, the evaporation which goes on from leaves, 

 especially from their under surfaces, is said to be a cause of the rise 

 of sap. It would be fairer to say, however, that rise of sap accom- 

 panies transpiration. For my birch when giving nearly a gallon of 

 sap a day had neither leaves nor catkins upon it, and when I first 

 noticed the outflow not a bud had burst. It follows, apparently, 

 that there may be flow of sap in the absence of transpiration, hence 

 that the one cause of the flow of sap is not transpiration. Doubt- 

 less transpiration plays an important part in the plant-growth, and, 

 as I understand, is so active at certain times as to be the possible 

 cause of such a reduction of pressure within a plant, as compared 

 with external atmospheric pressure, that so far fi'om any fluid being 

 exuded from a cut branch at such times, water may even be strongly 

 sucked in. At all events, so far as transpiration does affect the 

 flow of sap, such part of the flow would still seem to be caused only 

 by atmospheric pressure. 



There remains only to be considered the enormously powerful 

 attractive force termed the chemical force as lying at the bottom of 

 the attraction of plant-tissue for sap. In a plant the molecules 

 of carbonic acid, water, nitrogen-beai'ing bodies, and mineral sub- 

 stances are bound together by the chemical force into compounds, 

 and these latter into more complex compounds, and so the substance 

 of the plant or tree is formed. But the chemical force acts only 

 when bodies are in contact, that is, at insensible distances from each 

 other. How then can a root-tip obtain water or mineral matters ? 

 Water it will obtain from the molecules of water- vapour in contact 

 with the tip. At the tip the molecules will coalesce to drops and 

 these will dissolve contiguous molecules of mineral matter. Then 

 may come in capillary attraction, which is a variety of non-chemical 

 molecular attraction ; then may come in any influence of atmo- 

 spheric pressure whether set up by transpiration or otherwise ; then 

 may come in endosmose, which is indeed probably itself caused by 

 the chemical attraction between molecules of water and of saline 

 matter. 



But the chemical force is static rather than dynamic, how then 



