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 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. [Illustration: Fig. 35. Plants give out water through their leaves] Another thing you will notice is that fields lying at the side of a river and liable to be flooded, and fields {75} high up in wet hill districts, are covered with grass. In a clay country there is also a great deal of grass land and not much ploughed land; if you live where there is much clay you can easily discover the reason. Clay becomes very wet and sticky when rain falls, and very hard in dry weather: it is, therefore, difficult to cultivate. Farmers cannot afford to spend too much money on cultivation, and so they prefer grass, because once it is established it goes on indefinitely and does not want ploughing up and re-sowing. And besides, farmers have learned by experience that grass can tolerate more water and less warmth than most other English crops. There is much more grass land in those parts of England where the rainfall is high and the temperature rather {76} low--e.g. the northern parts of England--than in the eastern counties where the rainfall is low. [Illustration: Fig. 36. Stephen Hales's Experiment (from _Vegetable Staticks_, Vol. I. 1727)] The difference in water supply, therefore, leads us to expect the following differences between sandy soils and clays or loams:-- On sandy soils (the water content being small) the wild plants and trees usually have small leaves. Cultivated plants do not give very heavy crops, but they ripen early. On clay soils (the water content being good) wild plants and trees usually have larger leaves. Cultivated plants give good crops, but they ripen rather late. If the water content is too good or the clay is too sticky the land is generally put into grass. [Illustration: Fig. 37. Hill slope near Harpenden. Woodland at the top, arable land lower down. In the valley there is grass land but this is hidden by the cottages]