FORMATIONS. W? 



* The mechanical condition and physical peculiarities of 

 the soil will also be found to modify the effects of the 

 influential circumstance already referred to, and contribute 

 to chain particular plants to particular soils, or to facilitate 

 their dispersion. 



' There are plants which will only settle on unbroken 

 rocks, and which, when the other conditions coincide, spring 

 from these rocks to our walls, like the wall rue spleenwort 

 (Asplenium Ruta muraria), a little fern, the name of 

 which denotes its station. Others occur only where 

 weathering has broken up the solid rock into small frag- 

 ments drift plants, which, clinging to mankind, select 

 rubbish heaps, which most resemble their natural station : 

 the great nettle and the henbane serve as examples. 



' Lastly, other plants grow only where the rocks have 

 been reduced to fine powder in sand, or in the fine- 

 grained clay produced by chemical decomposition. The 

 so-called German sarsaparilla, the sea weed (Ammophila 

 arenaria), is an example of the first condition, but there is 

 no definite condition corresponding to it in the vicinity of 

 human habitations. 



' Clay, on the other hand, stands beside the black sub- 

 stance, humus, resulting from the decomposition of organic 

 matter. Both rich in soluble salts important to vegeta- 

 tion, both distinguished in their property of absorbing 

 from the atmosphere, and their conveying to the root of 

 plants, gases and aqueous vapour, they cause, single or in 

 combination, the most luxuriant vegetation. And we thus 

 obtain three stages in reference to the qualities of the soil 

 pure earths, wholly devoid of vegetation ; mixed earths, 

 without clay or humus, with an arid but characteristic 

 vegetation ; and lastly, soil rich in clay and humus, with 

 the greatest abundance and variety of plants. 



' Even in the north the eye of the uninstructed observer 

 is struck by the greater rich ness and stronger development 

 of the vegetable kingdom, from the argillaceous, basaltic, 

 and porphyritic soils ; and under the tropical sun, simple 

 quartz sand is a desert, if water, and therein foreign 



