ASCENDING SAP
“Now let us see how the plant is nourished by the various substances of
which we have just studied the most important. Every form of plant-life
is made up, not of a compact and uniform mass of matter with no
occasional empty spaces, but, on the contrary, with the aid of a
microscope it is seen that an infinite number of very minute cavities
called cells are interspersed throughout the body of the plant. These
cells may be regarded as extremely small closed sacs, sometimes round,
sometimes oval, but more often with irregular and angular outlines by
reason of the mutual pressure exerted by the cells. The cell-wall is
composed of an excessively fine membrane. In the pith of the elder, all
riddled like a sponge, you have an example of cells large enough to be
seen without a microscope. Other cavities are long, pointed at both
ends and swollen in the middle like a spindle. They are called fibers.
Still others form canals of uniform size throughout, as fine as a hair
and long enough to extend from the roots to the topmost leaves. These
canals are called ducts. Look closely at the cross-section of a very
dry vine-branch, and you will see a multitude of orifices into which it
would be possible to thrust a hair. Those are the openings into so many
broken ducts. Everything in the plant, absolutely everything—root,
stalk, wood, bark, leaves, flowers, fruit, seeds, no matter what—is
composed of a mass of cells, fibers, and ducts.
“That understood, let us consider the root of the plant. In its new
parts, at the tip-ends of its finest ramifications, tip-ends that we
have called spongioles, it is composed of cells just formed and
consequently tender and fitted for absorbing easily the moisture in the
soil. Spongioles, then, fill themselves much as sponges would do. That
done, conduits offer their services for conveying the liquid to the top
of the plant: they are the ducts just referred to, and comparable here
to the water-pipes in our own fountains. But if in fountains water runs
by its own weight, going from the highest to the lowest point, it is
not so with the liquid absorbed by the roots, a liquid running from
below upward. What then is the force that makes it ascend?
“This force is in the buds or, to speak more correctly, in the leaves.
Each leaf is the seat of an active evaporation whose object is to rid
the plant of the great quantity of water required for dissolving in the
soil and then conveying to the leaves the nutritive substances present
in the soil. This evaporation leaves a void in the cells that have
given up the evaporated water. But this void is immediately filled from
the neighboring cells, which give up their contents and receive in turn
the contents of the next lower layers. From cell to cell, from fiber to
fiber, from duct to duct, a similar transfer takes place at points
farther and farther away from the evaporating surface, until the
tip-ends of the rootlets are reached, where a continuous absorption
makes good the loss of moisture by evaporation. The process reminds one
somewhat of the working of our pumps, in which the piston leaves behind
it a void that is immediately filled by the water in the pipe, which in
its turn gets water from the bottom of the well. This liquid which
ascends in every plant, absorbed by the spongioles of the rootlets and
put in motion by the evaporation from the leaves, is called ascending
sap, or crude sap. The sap is called ascending because it passes from
below upward, from the roots to the branches; and it is called crude
because it has not yet undergone the preparation that will turn it into
the nutritive liquid of the plant. Thus we have learned our first
lesson, namely: ascending sap is carried especially to those parts of
the plant where buds are numerous, where leaves abound; it seeks by
preference the ends of the branches, where evaporation is most active.
“We know that the surface wood is the newest; it is formed of cells,
fibers, and ducts whose cavities are free and whose walls are
permeable. The interior wood is older; its cells, fibers, and ducts are
encrusted, stopped up, decrepit, out of use. The liquid accordingly
makes its way where circulation is possible, and ceases to flow where
the passage is obstructed. That is to say, the ascent of the sap takes
place through the sap-wood and chiefly through the outermost layers, or
those of most recent formation. Repeated experiment leaves no doubt on
this point. When a tree is cut down at the time of the sap’s greatest
activity, we find the sap-wood moist and the older wood perfectly dry.
Finally, in herbaceous plants the sap ascends through the whole body of
the stem. Suspended during the winter on account of the absence of
foliage, this ascent of the sap becomes remarkably brisk at the
awakening of vegetation. Then it is that fruit-trees shed tears, so to
speak, where the pruning-hook has left its mark; or, in other words,
the ascending sap oozes from the openings of the severed ducts. These
tears are especially noticeable in the grape-vine, where it has
recently been trimmed.
“Now what would you expect to find in this liquid if you collected some
of it as it trickles in the form of tears either from the vine or from
a fruit-tree? Many things, doubtless, you will say, since this precious
liquid is the prime source of all that the plant contains in itself. If
such is your thought, undeceive yourselves: ascending sap is little
more than clear water, and often it is very difficult for science to
prove beyond a doubt the presence in it of various substances in
solution, so minute a fraction of the whole do they compose. Among
these substances the most frequent are compounds of potash, of lime, of
carbonic acid gas, traces of phosphates, and compounds of nitrogen or
ammonia. In short, the liquid from which the plant is to derive its
nourishment is the weakest sort of broth, composed of an enormous
quantity of water and a very small proportion of dissolved substances.
These inconsiderable substances are the only or almost the only things
utilized by the plant; and the water that has collected them in
filtering through the soil, and has then carried them from the roots to
the leaves through the sap-wood, the water that forms almost the whole
of the ascending sap, is destined, as soon as the journey is
accomplished, to leave the plant and return as vapor to the atmosphere
whence it descended in the form of rain.”