(364 SCIENCE OF AGRICULTURE. Part II. 



different climates. It points out the grand features of the immense extent which plants 

 occupy, from the regions of perpetual snow to the bottom of the ocean, and to the 

 interior of the globe, where, in obscure grottoes, cryptogamous plants vegetate, as un- 

 known as the insects which they nourish. The superior limits of vegetation are known, 

 but not the inferior ; for every where in the bowels of the earth are germs which develope 

 themselves when they find a space and nourishment suitable for vegetation. On taking 

 a general view of the disposition of vegetables on the surface of the globe, independ- 

 ently of the influence of man, that disposition appears to be determined by two sorts of 

 causes, geographical and phi/sical. The influence of man, or of cultivation, has intro- 

 duced a third cause, which may be called civil. The different aspects of plants, in 

 different regions, has given rise to what may be called their characteristic or picturesque 

 distribution; and the subject of distribution may be also considered relatively to the system- 

 atic divisions of vegetables, their arithmetical proportions, and economical applications. 



Sect. I. Geographical Distribution of Vegetables. 



1688. The territorial limits to vegetation are determined in general by three diiVerent 

 causes : I. 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 mountains. 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 

 European 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 orga- 

 nised 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. 



1689. The natural circumstances affecting the distribution of plants, may be considered 

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



1690. Temjyerature has i\\e raost ohsioxxs influence on vegetation. Every one knows 

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

 trary. The wheat and barley of Europe will not grow within the tropics ; the same re- 

 mark 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 strong as to 

 stagnate the sap in the stems of plants ; and those where it freezes sufficiently strong to 

 penetrate into the cellular tissue ; form three classes of regions in which vegetation ought 

 to differ. But this difference is somewhat modified by the effect of vegetable structure, 

 which resists, in different degrees, the action of frost ; tlius, in general, trees wln'ch 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, v/hich are more favorably 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. 



1691. Some plants of firm texture, but natives of warm climates, udll endure a frost of 

 a few hours' continuance, as the orange at Genoa, ( Humboldt, De Distributione Flnn- 

 tarum) ; 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, nasturtium, &c. 



1692. I'he temperature of spring has a material influence on the life of vegetables ; the 

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

 favored 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. 



