SOME EXPERIMENTS ON THE SAP OF PLANTS. 263 
nitrogen ; a distinct trace of nitrites; 7°4 grains of nitrates con- 
taining 17 per cent. of nitrogen; no chlorides, or the merest trace ; 
no sulphates; no sodium salts; a little of potassium salts ; much 
phosphate and organic salts of calcium ; and some similar magnesian 
compounds. These calcareous and magnesian substances yielded 
an ash when the sap was evaporated to dryness and the sugar and 
other organic matter burnt away, the amount of this residual 
mineral matter being exactly 50 grains per gallon. The sap con- 
tained no peroxide of hydrogen. It was faintly if at all acid. 
Exposed to the air, it soon swarmed with bacteria, its sugar being 
changed to alcohol, and this again in a few days to acetic acid. 
The birch-sap had changed to birch-wine and then to birch-vinegar. 
It is noteworthy that the sap when drawn contained a ferment 
capable of transforming starch into sugar. Two former students of 
mine, Messrs. Dunstan and Dimmock, who have devised a method 
of ascertaining the power of such ferments, were good enough to 
apply their process to some of my sap. They found that one 
gallon would convert into sugar 21 grains of dry starch, the latter 
being first gelatinised. Here probably we get an idea of the 
methods which nature employs in converting one substance into 
another during the synthetical process or growth, and during the 
analytical process or more or less rapid decay or combustion of 
plants. Future investigation in physiological botany will doubtless 
include much experimenting with ferments. It is an extremely 
interesting branch of study. The action is well illustrated in the 
conversion of starch into gum, sugar, etc., during the germination 
of barley in the manufacture of malt, 
Shortly, respecting the composition of this sap, I may state to 
the general members of a Natural History Society without risking 
the dignity of my subject, that a teaspoonful of sugar put into a 
gallon of such rather hard well-water as we have in our chalky 
district, would very fairly represent this specimen of the sap of the 
silver birch. Indeed, in the phraseology of a water-analyst, I may 
say that the sap itself had 25 degrees of total, permanent hardness. 
How long the tree would continue to yield such a flow of sap I 
cannot say. Probably until the store of sugar it manufactured last 
summer to feed its young buds this spring was exhausted. Even 
within 48 hours the sugar slightly diminished in proportion in the 
fluid, the specific gravity going down from 100492 to 1004°38. 
And now with regard to the physics of the matter. What causes 
this outflow? Or, to put the larger question at once, what causes 
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. 
