THE DATE PALM is the most valuable food plant of the tropical desert, and the chief source of its timber supply. No degree of dry ness in the atmosphere and no amount of heat will injure its growth. Atmospheric humidity, indeed, is positively detrimental to it. On the other hand, it requires a continuous supply of moisture about its roots, but it is able to resist large quantities of alkali which is often present both in the soil and water of arid regions. Although the date palm is at certain seasons of the year able to exist with the thermometer below freezing point, it requires a very high temperature for the development of its fruit, and the lower zero point for flowering is believed to be 65.5°F. A mean temperature of during the fruiting season (May to October) with one month at least above 80°F. will enable early varieties of dates to ripen ; for later varieties the temperatures must be above 75°F. and respectively, and for the best and latest varieties and 94°F.
TEA grows in certain favoured localities of the wet evergreen and warm temperate forest regions. Being able to stand a greater range of temperature, it has a wider extension than either coffee or cacao, and in Asia it is grown as far north as the 45th parallel. The conditions most favourable to it are a temperature not falling for long below and not rising, except for short periods, above A rainfall of at least 60 inches is essential, and the best results are obtained when there are nearly 100 inches, a considerable part of which falls during the vegetative season. At the same time the land must be well-drained, and for this reason it was long thought that the tea plant would only flourish upon the lower slopes of hills ; but, within recent years, many gardens have been established in level country where the land is not liable to be water-logged. The plant thrives best on deep, reddish-coloured, sandy Mains with a free subsoil. Soils, rich in humus, are beneficial to it, and for this reason it grows well in districts which have been reclaimed from virgin forest, or even from swamps.
COTTON.—This plant belongs to the Malvacea or mallow family, the species of it which are most cultivated for commercial purposes being Gossypium herbaceum, Gossypium arboreum, and Gossypium barbadense. The first of these is allied to Gossypium hlrsutum, which in one or other of its many varieties is largely grown in the United States. From Gossypium barbadense Sea-island cotton is obtained. The bulk of the world's cotton supply is at present cultivated on lands which have been cleared of lighter tropical and warm temperate forests, on warm temperate savannas, and even on desert and semi-desert areas, when irrigation is practicable. Conditions of soil and climate restrict the growth of the plant even within these regions. Its lower zero points are relatively high, and six or seven months free from frost are necessary for its development. During the time that the plant is growing an increase in temperature is advantageous, but, after full vegetative growth has been attained, a decrease in temperature and an increas ing diurnal range prevent the plant from running to wood, and cause it to devote the food supply which it has accumulated to the nourishment of its seed. The optimum temperatures for the various functions have not yet been carefully studied. Accord ing to Heuze, the most favourable daily temperature from germina tion to flowering is from to and from flowering to maturity, to The moisture required for the development of the plant is sup plied, as will be seen later, under conditions which vary so much in different parts of the world, that no absolute statement regarding the amount necessary can be made. In the earlier stages of its growth frequent precipitations, increasing in volume with the progress of the plant, yield the most satisfactory results. The absence of a sufficient supply of moisture at this period leads to stunted growth and premature ripening, while too great an abundance of it gives rich vegetation with little fruit. Later on a decreasing rainfall is favourable to the seed maturing.