Besides sugar, which is present in this sap to the extent of 616 grains--nearly an ounce and a half--per gallon, there are present a mere trace of mucilage; no starch; no tannin; 3½ grains per gallon of ammoniacal salts yielding 10 per cent. of nitrogen; 3 grains of albuminoid matter yielding 10 per cent. of nitrogen; a distinct trace of nitrites; 7.4 grains of nitrates containing 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 yield an ash when the sap is evaporated to dryness and the sugar and other organic matter burnt away, the amount of this residual matter being exactly 50 grains per gallon. The sap contained no peroxide of hydrogen. It was faintly if at all acid. It held in solution a ferment capable of converting starch into sugar. Exposed to the air it soon swarmed with bacteria, its sugar being changed to alcohol.
A teaspoonful or two of, say, apple juice, and a tablespoonful 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 has 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 twenty-four hours the sugar has slightly diminished in proportion in the fluid.
Whether or not this little note throws a single ray of light on the much debated question of the cause of the rise of sap in plants I must leave to botanists to decide. I cannot hope that it does, for Julius Sachs, than whom no one appears to have more carefully considered the subject, says, at page 677 of the recently published English translation of his textbook of botany, that "although the movements of water in plants have been copiously investigated and discussed for nearly two hundred years, it is nevertheless still impossible to give a satisfactory and deductive account of the mode of operation of these movements in detail." As a chemist and physicist myself, knowing something about capillary attraction, exosmose, endosmose, atmospheric pressure, and gravitation generally, and the movements caused by chemical attraction, I am afraid I must concur in the opinion that we do not yet know the real ultimate cause or causes of the rise of sap in plants.
Ashlands, Watford, Herts.
THE CROW.
[Footnote: Abstract of a recent discussion before the Connecticut State Board of Agriculture.]
Prof. W. A. Stearns, in a lecture upon the utility of birds in agriculture, stated that the few facts we do know regarding the matter have been obtained more through the direct experience of those who have stumbled on the facts they relate than those who have made any special study of the matter. One great difficulty has been that people looked too far and studied too deeply for facts which were right before them. For instance, people are well acquainted with the fact that hawks, becoming bold, pounce down upon and carry off chickens from the hen-yards and eat them. How many are acquainted with the fact that in hard winters, when pressed for food, crows do this likewise? But what does this signify? Simply that the crow regulates its food from necessity, not from choice.