Fig. 33 shows two pots of wheat, one kept only just sufficiently moist for growth, the other kept very moist but not too wet. You can see what a difference there is; in the drier pot the leaves are rather narrow and the plants are small, in the moister pot the leaves are wide and the plants big. But there was also another difference that the photograph does not bring out very well--the plants in the rather dry soil were, as you can see, in full ear, ripe and yellow, while those in the very moist soil were still green and growing. We see then (1) that on moist soils there is greater growth than on dry soils, but the plants do not ripen so quickly; (2) in very wet soils mustard--and many other plants also--will not grow. Water is not itself harmful. It is easy to grow many plants in water containing the proper food, but _air must be blown through the water at frequent intervals_. In the water-logged soil of Pot 15 the trouble arose not from too much water but from too little air. Air is wanted because plants are living and {71} breathing in every part, in the roots as well as in the leaves. [Illustration: Fig. 33. This wheat growing on very moist soil was still green and growing vigorously, whilst this wheat growing on rather dry soil was yellow and ripe] 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 {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]