240 



" The watery vapour of the atmosphere must, therefore, be brought 

 to the plant in some other way, and this happens through the pro- 

 perty of absorbing the moisture of the atmosphere, which is possessed 

 by most of the constituents of the soil. No substance possesses this 

 property in so high a degree as the humus, originating out of the 

 gradual decomposition of organic matters. The humus is also re- 

 markably distinguished for its special power of extracting, and as it 

 were collecting the carbonic acid and ammoniacal gas of the air ; no 

 solid substance of the soil equals it in this particular, and water itself 

 only ranks second after it. Humus consequently contains, under all 

 circumstances, water impregnated with carbonic acid and ammonia, 

 and in proportion as this is withdrawn from it by the roots of the 

 plants, the loss is replaced out of the atmosphere. This is certainly 

 the principal road by which water is conveyed into the plant, most 

 probably the most essential canal through which it is fed with am- 

 monia, and there is no doubt that at least a great portion of the car- 

 bonic acid is thus brought to it." — p. 161. 



The progress of vegetation, from the earliest appearance of plants 

 in their simplest possible form, up to the most complicated structure, 

 is well pourtrayed in the following extract, which also exhibits the 

 mode in which humus or vegetable mould is gradually accumulated 

 upon the previously bare surface of rocks, until a rich soil is formed, 

 capable of supporting a luxuriant vegetation. 



" Look at a recently exposed surface of a block of granite, for in- 

 stance, on the summit of the Brocken ; there we find that vegetation 

 is soon developed, in the form of a little delicate plant, which re- 

 quires the microscope for its recognition ; and this is nourished by 

 the small quantity of atmospheric water impregnated with carbonic 

 acid and ammonia. This, the so-called violet-stone, a scarlet, pul- 

 verulent coating over the bare stone, which, on account of the pecu- 

 liar smell of violets which it emits when rubbed, has become a cu- 

 riosity, industriously sought by the thoughtful wanderer on the 

 Brocken. By the gradual decay and decomposition of this little 

 plant, a very thin layer of humus is by degrees produced, which now 

 suffices to procure from the atmosphere food sufficient for a couple 

 of great blackish brown lichens. These lichens, which densely 

 clothe the heaps of earth round the shafts of the mines of Fahlun 

 and Dannemora, in Sweden, and through their gloomy colour, which 

 they impress on all around, make those pits and shafts look like the 

 gloomy abysses of death, have been appropriately called by botanists 

 the Stygian and Fahlun lichens. But they are no messengers of 



