ts 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]

Plants require to be sufficiently warm. Some like tropical heat and
can only be grown in hot houses; others can withstand a certain amount
of cold and will grow up on the mountains. Our common cultivated crops
come in between and will not grow in too cold or exposed a situation;
thus you find very little cultivated land 800 ft. above sea level, and
not usu