gravel ground there was nothing like
as great a growth of grass or of other plants as on the moister soil.
This is so much like what we found in the pot experiments that we shall
not be wrong in supposing that the difference in water supply largely
accounted for the difference in growth. But you may also have noticed
something else. Plants in the drier soil have generally {72} narrow
leaves and the grasses are rolled up and fine, whilst those on the damp
soil, including the grasses, have usually broad leaves. Thus in the
dry sandy soil you may find broom, spurrey, sheep's fescue, pine trees,
all with narrow leaves; whilst on the moister soil you may find
burdock, primroses, cocksfoot and other broad-leaved plants. Figs. 34
_a_ and _b_ show some plants we found on a dry, gravelly patch on
Harpenden common, and on a moist loam in the river valley below.

[Illustration: Fig. 34 _a_. Plants collected on dry sandy soil.
Broom, sheep's fescue, crested dogstail and gorse, all with narrow
leaves]

{73}

Before we can account for this observation, we must ascertain a little
more closely what becomes of the water the plant takes up. It
certainly does not all stay in the plant, and the only way out seems to
be through the leaves. Put a test tube on the leaf of a growing plant
and fix a split cork round the stem: leave in sunlight for a few hours
and notice that water begins to collect in the test tube (Fig. 35).
The experiment shows that water passes out of the plant through the
leaves.

[Illustration: Fig. 34 _b_. Plants collected on moist loam. All have
wide leaves]

This experiment was first made by Stephen Hales, and described by him
thus in 1727: "Having by many {74} evident proofs in the foregoing
experiments seen the great quantities of liquor that were imbibed and
perspired by trees, I was desirous to try if I could get any of this
perspiring matter; and in order to do it, I took several glass chymical
retorts, _b a p_ [Fig. 36] and put the boughs of several sorts of
trees, as they were growing with their leaves on, into the retorts,
stopping up the mouth _p_ of the retorts with bladder. By this means I
got several ounces of the perspiring matter of vines, figtrees"--and
other trees, which "matter" Hales found to be almost pure water. The
test tube experiment should now be made with a narrow-leaved grass like
sheep's fescue and with a wide-leaved grass like cocksfoot. You will
find that wide-leaved plan