3.  The sub-tropical zone extends from the tropics 23 to 34 deg. of latitude.  There are a number of tropical fruits in this region.  The winters are mild and vegetation is green throughout the year.  In the northern division of the zone palms and bananas grow on the plains.  In this region is comprised all the extreme northern portions of Africa, coasting the Mediterranean, comprising Algiers and the Barbary States, Egypt, part of Persia, Cabool and the Punjab; the greater portion of China, Lower California, Texas, the South-Western States of America, the Bermudas, the Cape Colony and Natal, New South Wales, Southern and Western Australia—­the Government settlements in the Northern Island of New Zealand, the largest portion of Chile, Paraguay, Uruguay and the Argentine Republics, the Provinces of Brazil from St. Paul to Rio Grande, Madeira and the Canary Isles.

To define accurately the conditions of temperature which a plant requires to maintain it in a flourishing condition we must ascertain within what limits its period of vegetation, may vary, and what quantity of heat it requires.  This most remarkable circumstance was first observed by Boussingault, but unfortunately we do not as yet possess sufficiently accurate accounts of the conditions of culture in the various regions of the earth, to enable us to follow out this ingenious view in all its details.  His theory is, that the time required by a plant to arrive at maturity is as the inverse ratio of the temperature; therefore, knowing the mean temperature of any place, and the number of days which a plant takes to ripen, the time required at any other point more or less elevated, can easily be ascertained.  Peter Purry, a native of Switzerland, who settled in Charleston in the eighteenth century, in a memorial to the Duke of Newcastle, then Secretary of State, sets out with this postulate, that “there is a certain latitude on our globe, so happily tempered between the extremes of heat and cold, as to be more particularly adapted than any other for certain rich productions of the earth; among which are silk, cotton, indigo,” &c.—­and he fixes on the latitude of 33 deg., whether north or south, as the one of that peculiar character.

The following Table, showing the climate, duration and production of certain plants cultivated in tropical America, is from the proceedings of the Agricultural Society of Grenada.  The second, column gives the altitude in English yards above the level of the sea.  The third, the mean temperature by Fahrenheit’s thermometer.  The fourth, the average time required to commence bearing.  The fifth, the number of plants in a Spanish “fanegada” of 170 varras, about 153 square yards.  The sixth, the average duration of each plant.  The seventh, the average produce of each plant in the year:—­