RESPONSE OF THE PLANT TO ITS SURROUNDINGS 291 
dog fennel (Anthemis cotula) which formerly held undisputed 
possession of arid places throughout the South Atlantic states. 
A still more remarkable instance is the invasion of the Japa- 
nese honeysuckle (Lonicera Japonica), originally introduced 
for ornament, but which has naturalized itself within the last 
thirty years and overrun waste places everywhere, from the 
Gulf to the Potomac, with a vigor and luxuriance equaled 
by few of our native species. As its beauty and fragrance 
are even more conspicuous in a state of nature than under 
cultivation, and as it can, moveover, be made very useful in 
stopping gullies and washes, its phenomenally rapid occu- 
pation of so large a territory has caused no alarm and 
consequently attracted little attention. 
329. Climatic zones.—— These are more general group- 
ings than those we have been considering. They follow 
in a rough way the parallels of latitude, and are classed 
accordingly as: (1) tropical; (2) subtropical; (3) temperate ; 
(4) boreal or (on mountains) subalpine; (5) arctic or (on 
high mountains) alpine. Taking the cultivated plants of 
our own country by way of illustration, we have the sub- 
tropical zone, embracing Florida and the southern portion 
of the Gulf states, where sugar cane, rice, and tropical 
fruits are the staple crops. Then comes the temperate 
zone, with three agricultural subdivisions: (a) the great 
cotton belt, with Indian corn, sweet potatoes, and the 
peach, melon, and fig as secondary products. Farther 
north, in the Central and Middle Atlantic states, we find 
(b) the region of maize, hemp, and tobacco, with grapes, 
apples, pears, cherries, and a great variety of garden vege- 
tables as side crops. Finally comes (c) the great wheat- 
growing region of the North, with buckwheat, hay, and Irish 
potatoes as subsidiary crops. 
Technically, the distribution of the natural zones of vege- 
tation from south to north is classed under the three general 
heads of Forest, Grass Land, and Arctic Desert, with numer- 
ous subdivisions in each. 
