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 usually 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 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.
