Now turn to what you have seen in your walks. You would probably notice that on the drier, sandy or 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 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.
Fig. 34 a. Plants collected on dry sandy soil. Broom, sheep's fescue, crested dogstail and gorse, all with narrow leaves
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.
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 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 plants pass out more water than those with narrow leaves, and hence wide-leaved plants occur in damp situations or on damp soils like loams and clays, while narrow-leaved plants can grow on dry, sandy soils.