[Footnote: Read at an evening meeting of the Pharmaceutical Society,
London, April 4, 1883.]
By Professor ATTFIELD, F.R.S.
Beneath a white birch tree growing in my garden I noticed, yesterday
evening, a very wet place on the gravel path, the water of which was
obviously being fed by the cut extremity of a branch of the birch about
an inch in diameter and some ten feet from the ground. I afterward found
that exactly fifteen days ago circumstances rendered necessary the
removal of the portion of the branch which hung over the path, 4 or 5
feet being still left on the tree. The water or sap was dropping fast
from the branch, at the rate of sixteen large drops per minute, each
drop twice or thrice the size of a "minim," and neither catkins nor
leaves had yet expanded. I decided that some interest would attach to a
determination both of the rate of flow of the fluid and of its chemical
composition, especially at such a stage of the tree's life.
A bottle was at once so suspended beneath the wound as to catch the
whole of the exuding sap. It caught nearly 5 fluid ounces between eight
and nine o'clock. During the succeeding eleven hours of the night 44
fluid ounces were collected, an average of 4 ounces per hour. From 8:15
to 9:15 this morning, very nearly 7 ounces were obtained. From 9:15
to 10:15, with bright sunshine, 8 ounces. From 10:15 until 8:15 this
evening the hourly record kept by my son Harvey shows that the amount
during that time has slowly diminished from 8 to a little below 7 ounces
per hour. Apparently the flow is faster in sunshine than in shade, and
by day than by night.
It would seem, therefore, that this slender tree, with a stem which at
the ground is only 7 inches in diameter, having a height of 39 feet,
and before it has any expanded leaves from whose united surfaces large
amounts of water might evaporate, is able to draw from the ground about
4 liters, or seven-eighths of a gallon of fluid every twenty-four hours.
That at all events was the amount flowing from this open tap in its
water system. Even the topmost branches of the tree had not become,
during the fifteen days, abnormally flaccid, so that, apparently, no
drainage of fluid from the upper portion of the tree had been taking
place. For a fortnight the tree apparently had been drawing, pumping,
sucking--I know not what word to use--nearly a gallon of fluid daily
from the soil in the neigborhood of its roots. This soil had only an
ordinary degree of dampness. It was not wet, still less was there any
actually fluid water to be seen. Indeed, usually all the adjacent soil
is of a dry kind, for we are on the plateau of a hill 265 feet above the
sea, and the level of the local water reservoir into which our wells dip
is about 80 feet below the surface. My gardener tells me that the tree
has been "bleeding" at about the same rate for fourteen of the fifteen
days, the first day the branch becoming only somewhat damp. During the
earlier part of that time we had frosts at night, and sunshine, but with
extremely cold winds, during the days. At one time the exuding sap
gave, I am told by two different observers, icicles a foot long. A much
warmer, almost summer, temperature has prevailed during the past three
days, and no wind. This morning the temperature of the sap as it escaped
was constant at 52° F., while that of the surrounding air was varying
considerably.
The collected sap was a clear, bright, water-like fluid. After a pint
had stood aside for twelve hours, there was the merest trace of a
sediment at the bottom of the vessel. The microscope showed this to
consist of parenchymatous cells, with here and there a group of
the wheel-like or radiating cells which botanists, I think, term
sphere-crystals. The sap was slightly heavier than water, in the
proportion of 1,005 to 1,000. It had a faintly sweet taste and a very
slight aromatic odor.
Chemical analysis showed that this sap consisted of 99 parts of pure
water with 1 part of dissolved solid matter. Eleven-twelfths of the
latter were sugar.
That the birch readily yields its sap when the wood is wounded is well
known. Philipps, quoted by Sowerby, says:
"Even afflictive birch,
Cursed by unlettered youth, distills,
A limpid current from her wounded bark,
Profuse of nursing sap."
And that birch sap contains sugar is known, the peasants of many
countries, especially Russia, being well acquainted with the art of
making birch wine by fermenting its saccharine juice.
But I find no hourly or daily record of the amount of sugar-bearing
sap which can be drawn from the birch, or from any tree, before it
has acquired its great digesting or rather developing and transpiring
apparatus--its leaf system. And I do not know of any extended chemical
analysis of sap either of the birch, or other tree.
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.
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