Book I. DISTRIBUTION OF VEGETABLES. 265 



Sect. I. Geographical Distribution of Vegetables. 



1723. The territorial limits to vegetation are determined in general by three causes : 

 1. By sandy deserts, which seeds cannot pass over either by means of winds or birds, as 

 that of Sahara, in Africa ; 2. By seas too vast for the seeds of plants to be drifted from 

 one shore to the other, as in the ocean ; while the Mediterranean sea, on the contrary, 

 exhibits the same vegetation on both shores ; and, 3. By long and lofty chains of moun- 

 tains. To these causes are to be attributed the fact that similar climates and soils do 

 not always produce similar plants. Thus in certain parts of North America, which 

 altogether resemble Europe in respect to soil, climate, and elevation, not a single Eu- 

 ropean plant is to be found. The same remark will apply to New Holland, the Cape 

 of Good Hope, Senegal, and other countries, as compared with countries in similar phy- 

 sical circumstances, but geographically different. The separation of Africa and South 

 America, Humboldt considers, must have taken place before the developement of 

 organised beings, since scarcely a single plant of the one country is to be found in a wild 

 state in the other. 



Sect. II. Physical Distribution of Vegetables. 



1724. The natural circumstances affecting the distribution of plants may be considered 

 in respect to. temperature, elevation, moisture, soil, and light. 



1725. Temj)erature has the most obvious influence on vegetation. Every one knows 

 that the plants of hot countries cannot in general live in such as are cold, and the contrary. 

 The wheat and barley of Europe will not grow within the tropics. The same remark 

 applies to plants of still higher latitudes, such as those within the polar circles, which 

 cannot be made to vegetate in more southern latitudes ; nor can the plants of more southern 

 latitudes be made to vegetate there. In this respect, not only the medium temperature 

 of a country ought to be studied, but the temperature of different seasons, and especially 

 of winter. Countries where it never freezes, those where it never freezes so strongly as 

 to stagnate the sap in the stems of plants, and those where it freezes with strength suflS- 

 cient to penetrate into the cellular tissue, form three classes of regions in which vege- 

 tation ought to differ. But this difference is somewhat modified by the effect of vegetable 

 structure, which resists, in different degrees, the action of frost. Thus, in general, trees 

 which lose their leaves during winter resist the cold better than such as retain them ; 

 resinous trees, more easily than such as are not so ; herbs of which the shoots are 

 annual and the root perennial, better than those where the stems and leaves are persisting; 

 annuals which flower early, and whose seeds drop and germinate before winter, resist 

 cold less easily than such as flower late, and whose seeds lie dormant in the soil till spring. 

 Monocotyledonous trees, which have generally persisting leaves and a trunk without 

 bark, as in palms, are less adapted to resist cold than dicotyledonous trees, which are 

 more favourably organised for this purpose, not only by the nature of their proper juice, 

 but by the disposition of the cortical and alburnous layers, and the habitual carbonisation 

 of the outer bark. Plants of a dry nature resist cold better than such as are watery ; 

 all plants resist cold better in dry winters than in moist winters ; and an attack of frost 

 always does most injury in a moist country, in a humid season, or when the plant is too 

 copiously supplied with water. 



1726. Some plants of firm texture, but natives of warm climatesy vM endure a frost of 

 a few hours' continuance, as the orange at Genoa, {Humboldt, De Distributione Planta- 

 rum) ; and the same thing is said of the palm and pine-apple, facts most important for 

 the gardener. Plants of delicate texture, and natives of warm climates, are destroyed 

 by the slightest attack of frost, as the Phaseolus, iVasturtium, &c. 



1727. The temperature ofspringhas a material influence on the life of vegetables ; the 

 injurious effects of late frosts are known to every cultivator. In general, vegetation is 

 favoured in cold countries by exposing plants to the direct influence of the sun ; but this 

 excitement is injurious in a country subject to frosts late in the season ; in such cases, it 

 is better to retard than to accelerate vegetation. 



1728. The temperature of summer, as it varies only by the intensity of heat, is not pro- 

 ductive of so many injurious accidents as that of spring. Very hot dry summers, how- 

 ever, destroy many delicate plants, and especially those of cold climates. A very early 

 summer is injurious to the germination and progress of seeds ; a short summer, to their 

 jipening, and the contrary. 



1729. Autumn is an important season for vegetation, as it respects the ripening d. 

 seeds ; jience where that season is cold and humid, annual plants, which naturally flower 

 late, are never abundant, as in the polar regions ; the eflfect is less injurious to perennial 

 plants, which generally flower earlier. Frosts early in autumn are as injurious as those 

 which happen late in spring. The conclusion, from these considerations, obviously is, 

 that temperate climates are more favourable to vegetation than such as are either extremely 

 cold or extremely hot : but the warmer climates, as Keith observes, aie more favourable, 



