GEOGRAPHICAL DISTRIBUTION OF PLANTS. 



91 



CHAPTER XXXII. 



GEOGEAPHIOAL DISTRIBUTION OF PLANTS. 



520. MATERIALS of this very interesting and impor- 

 tant branch of the science have been gathering since the 

 days of the early Botanists ; for among the scientific ob- 

 servations which they made and recorded, was that of the 

 place where a new plant was found. In the year 1700, 

 Tourneforte, who had been sent to Greece and Asia Mi- 

 nor for the purpose of scientific observation and discovery, 

 noticed, while ascending Mount Ararat, that gradations 

 of temperature occurred, similar to what he had observed 

 in travelling from the Levant to Lapland ; and in that 

 observation the great fact was developed, that height 

 above the sea has the same effect on temperature as dis- 

 tance from the equator ; and hence that the mountains 

 of torrid climes will exhibit the climatic conditions of 

 every zone. About the year 1806, Humboldt, while in- 

 vestigating the relations between the distribution of 

 plants and the temperature of climates, conceived the 

 ingenious idea of connecting all those countries of the 

 same mean temperature, by what he termed ISOTHERMAL 

 LINES, or lines of equal heat. He found that while they 

 sometimes diverge greatly from lines of latitude, they 

 form far better boundaries of vegetation than any lines 

 which merely indicate distance from the equator. About 

 the same time, Adanson observed that there are few, if 

 any, Umbelliferous plants in the tropics ; and from this, 

 observations continued to be made, with still increasing 

 accuracy, until the earth was mapped out into distinct 

 regions, or Floras, which often bear the names of their 

 definers. Of these let us now take a brief survey. 



521. Of the 1 10,000 plants known, the greater portion 

 are indigenous to tropical America. Europe contains 

 about half the number ; Asia, with its islands, less than 

 Europe ; New Holland and the Pacific islands still less ; 

 and Africa fewest of all. One curious fact has been ob- 

 served, that similar forms, yet not identical species, are 

 often found in corresponding latitudes of different coun- 

 tries. Thus the Cacti of tropical America, ' not a single 

 species of which is indigenous to the Old World, are re- 

 presented by the leafless Spurge-worts of the correspond- 

 ing parched and torrid regions in the East. The Euro- 

 pean Daisy is not to be found in all America, while its 

 place is supplied by various species of Aster, some of 



General subject. How long have its materials been accumulating? What 

 observation generally recorded ? Observation of Tourneforte when made ? 

 Define. Idea of Humboldt define when conceived ? Observation of Adan- 

 son. What progress since ? How many plants known ? Of these how many 

 found in Tropical America Europe Asia and its islands New Holland and 

 the Pacific islands Africa ? What curious fact observed? Instances. Of 

 which continent is Heath a native ? What nearly allied genus where found ? 



them closely resembling it, and belonging to the same 

 Order. Again, the Heath-tribes overspread the whole 

 Eastern continent, only leaping over the torrid portion, 

 from the southern point of Africa to the frigid region of 

 the North Cape ; while not a single species is to b 

 found in all America. Yet most others of the genera 

 belonging to the same Order (Ericaceae) are far more 

 numerous than they are in the Old World. Several of 

 these are peculiar to the New ; and one tribe in particular 

 the Whortleberry tribe is very widely and profusely 

 scattered. Islands generally have the vegetation of the 

 nearest continents ; but if far from the mainland, they are 

 marked by a peculiar vegetation. St. Helena, standing 

 alone in the midst of the ocean, out of sixty -one indige- 

 nous plants, has only two or three that are to be found 

 elsewhere. Few social plants, such as the Grasses and 

 Heaths, are to be met with in the torrid zone. The most 

 favorable conditions of the Grape are found between 30 

 and 45 of north latitude, and there the best wines are 

 made. Wheat is cultivated as far north as 60 5 but in 

 the torrid zone, it will not form an ear at less than 4500 

 feet above the sea. Different seas also have their pecu- 

 liar vegetation. Such vast quantities of a certain species, 

 called Gulf-weed, are produced in the Gulf of Mexico, 

 that banks are formed, through which it is difficult for 

 ships to pass ; and shallow seas have different plants 

 from those which are deeper and colder. 



522. THE ARCTIC FLORA (Eegion of Wahlenberg). 

 This comprehends all the countries within the Polar 

 circle. It extends from the region of eternal snows, or 

 the summits of high mountains, down to the limits of the 

 growth of trees, and is distinguished by the tufted forms 

 of its plants, which often have large and conspicuous 

 flowers, as the Poppy and Crowfoot, and by the entire 

 absence of trees. In the extreme north of this region, 

 the dreary snow-plains are sometimes overspread for 

 miles with the Protococcus, whose carmine bloom tinges 

 the sparry surface with the warm glow of sunset. Here, 

 too, before the short summer appears, other plants are 

 seen vegetating under the snow. They melt the snow in 

 immediate contact with themselves, the upper strata of 

 which being thus converted into a transparent sheet of 

 ice, through which the beams of the sun shine freely, 

 form a kind of natural hothouse. Thus protected from 

 the fiercer cold, the plants continue to flourish until the 



What vegetation have islands generally when far from main land St. Helena? 

 In what zone are few social plants ? Most favorable condition of the Grape ? 

 How far north is Wheat cultivated ? How low on mountains of the torrid zone 

 will it form an ear ? Do all seas have the same kind of vegetation ? What 

 of the Gulf of Mexico ? What other differences ? The Arctic Flora whose re- 

 gion? Define its limits by what distinguished ? What red-flowering plant? 

 Natural hothouses, 



