THE GEOGRAPHY OF FRUIT GROWING 619 



The Tropical Zone. — Only a very small part of the continental United 

 States is included within the Tropical region or zone. To be exact, there 

 are three widely separated areas where tropical conditions prevail and 

 tropical vegetation abounds — one in southern Florida, one in extreme 

 southeastern Texas, and one along the California-Arizona line, extending 

 as far north as southern Nevada. Within these areas are such fruits as 

 the banana, pineapple, mango, date palm, cocoanut, papaya and cheri- 

 moya. This region is never visited by frosts or freezing temperatures and 

 many of the fruits grown in it are said to be seriously injured by tempera- 

 tures even closely approaching the freezing point. To the pomologist, 

 as to the biologist, this region is known as the Tropical zone. It is char- 

 acterized by having more than 14,400°C. (26,000°F.) of heat during the 

 year — degrees of normal mean daily heat in excess of a minimum of 6°C. 

 (43°F.),*^ which is rather arbitrarily assumed as marking the inception 

 of physiological activity in plants. 



Austral or Temperate Zone. — Between the Boreal region on the north 

 and the Tropical region on the south and embracing most of the area of the 

 United States, is a region designated as Austral on the maps of biological 

 surveys and designated as the Temperate zone by the pomologist. Frosts 

 and freezes are likely to occur throughout most of this region, but mini- 

 mum winter temperatures seldom go below — 30°F. at the north and the 

 mean temperature of midwinter months even of the more northern 

 sections is well above zero. 



Transition Zone. — Biologists recognize three transcontinental life 

 zones within this region, a so-called Transition zone to the north and an 

 Upper Austral and Lower Austral zone to the south. Some of the more 

 hardy fruits, as the apple, pear, red raspberry and the Nigra and European 

 groups of plums find their most congenial home in the Transition zone. 

 In the east this zone includes most of those portions of New England, 

 New York, Pennsylvania and Michigan and in the middle west most of 

 those portions of Wisconsin, Minnesota and the Dakotas not included 

 in the Boreal region; in the west it includes many irregularly shaped 

 areas from the Canadian border to Mexico, and even in Mexico, where 

 elevation causes comparatively low temperatures. "Transition zone 

 species", Merriam states, ''require a total quantity of heat of at least 

 5500°C. (10,000°F.) but can not endure a summer temperature the mean 

 of which for the six hottest weeks exceeds 22°C. (71.6°F.) The northern 

 boundary of the Transition zone, therefore, is marked by the isotherm 

 showing a sum of normal positive temperatures of 5,500°C. (10,000°F.), 

 while its southern boundary is coincident with the isotherm of 22°C. 

 (71.6°F.) for the six hottest consecutive weeks. "''^ 



This Transition zone is in turn divided into three areas by lines having 

 a general north and south direction, areas that differ from one another 

 primarily in rainfall and atmospheric humidity. The eastern area, 



