ally much above 500 ft. At this height it is left as grass
land, and higher up as woodland, moor, or waste land. Grass requires
less warmth and can therefore grow at greater heights than many other
crops. If you start at the top of a hill in Derbyshire, and walk down,
you will see that the top is moorland, lower down comes grass land,
still lower you may find arable land, and if the valley is damp you
will find more grass at the bottom. Figs. 37 and 38 show typical views
of the hill slopes further south: they are taken near Harpenden. The
top of the hill in each case is over 400 ft. above sea level, and has
never been thought worth cultivating, but has always been left as {78}
wood because it is too exposed for farm crops. On the lower slopes the
arable fields are seen, while at the bottom bordering the river is
rough grass land, shown in Fig. 39. The top is too cold and windy, and
the bottom too wet, to be worth cultivating.

[Illustration: Fig. 38. View further along the valley, woodland and
arable above rough grass land near the river]

As the plant root is alive it wants air. The effect of keeping air out
can be seen by sowing some barley or onion seeds in the ground and then
pouring a lot of water on and plastering the soil down with a spade.
Sow another row in nicely crumbled soil, not too wet, press the seeds
well in, but do not plaster the soil. This second lot will generally
do much better than the first. If the ground round a plant is
frequently trodden so that it becomes very hard the plant makes much
less growth than if the soil were kept nice and loose. A good gardener
takes very great pains in preparing his ground before he sows his
seeds, and he is careful that no one should walk on his beds lest his
plants should suffer.

SUMMARY. We may now collect together the various things we have learnt
in this chapter. Plants require water, air, warmth, food, and light,
and they will not grow if harmful substances are present. The
rain-water that falls remains for some time in the soil, and does not
at once run away or dry off: water can also move from wet to dry places
in the soil. Therefore the plant does not need rain every day, but can
draw on the stock in the soil during dry weather. A sandy soil is
usually drier than a loam or a clay, especially if it lies rather high:
plants growing on a sandy soil make less growth and have narrower and
smaller leaves than those on a moister soil.

Situa