SOME EXPERIMENTS ON THE SAP OF PLANTS. 265 
yielding nearly a gallon of sap daily when no wind was blowing, 
and the amount was not perceptibly 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 nature abhorring a vacuum 
drives in water to supply the thus enlarged space or spaces. How 
any such warmth expands the solid without at the same time 
expanding the liquid 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 which 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 from 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-bearing bodies, and mineral sub- 
stances are bound together by the chemical force into compounds, 
and these latter into more complex compounds, and so the sub- 
stance 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 
