Esta hUshment of Vegetation at the Surface of the Globe. 69 



bles suitable to the temperature of the localities, provided they 

 be frequently watered by rain. 



The circumstances which subject sand to the power of vege- 

 tation do not everywhere exist ; there are even vast countries 

 where the earth appears condemned to present to its inhabitants 

 nothing but a dry and burnt surface. Such are those immense 

 plains of Africa, those dreadful deserts, the countries of silence 

 and of death, which man traverses only with fear, but which Na- 

 ture may yet, by means of certain local circumstances, bring to 

 a state of life, as she has done in many other places. The most 

 efficacious, in fact the only means of doing this, is the presence 

 of water. We already know, that several great rivers carry 

 their waters through them, such as the Nile in Egypt, and the 

 Niger in a part of the Sahara. The springs which feed them, 

 enlarged by the rains, occasion, every year, considerable over- 

 flowings. These superabundant waters deposit, upon the lands 

 which have been inundated by them, a mud which, by being 

 mingled with sand, acquires a great degree of fertility ; in other 

 places they form seas, lakes, and pools, which carry the princi- 

 ples of life into those countries of death. 



A new order of plants meets us upon the edges, and at the 

 surface of these lakes. We can easily imagine, that those which 

 have established vegetation upon the sandy or stony soils could 

 not here fulfil the same object, and we shall see this all-powerful 

 Nature overcoming with time, the obstacles which oppose them- 

 selves to its operation. When the waters have covered a piece 

 of ground, plants almost immediately begin to appear ; they are 

 more or less abundant, according to circumstances If these 

 waters are running hke those of rivers, or agitated like those of 

 great lakes, vegetation only exists upon their edges ; but if they 

 be tranquil, stagnant, and of little depth, plants grow in them 

 more numerously, and with more rapidity ; they at first cover 

 the surface of the waters, and occupy, from the simplicity of 

 their organisation, the same order as those which grow upon 

 rocks ; they are merely very delicate, interwoven filaments, with- 

 out roots, and without apparent fructification. They precede 

 the growth of more perfect vegetables, and prepare the soil 

 which is to receive them, — an operation which we may equal- 

 ly observe without leaving our houses. If we examine neglect- 



