150 l>Ii- SCHLEIDEN'S TIIEOltY OK AGRICULTURE. 



cause. Many mosses and liverworts can be bvonglit to fructify 

 under cultivation only when covered with green glass, which 

 imitates the peculiar light of the places in which tliey naturally 

 grow. 



Electricity, beyond doubt, is a powerful agent in vegetation, 

 but at present we are not in a condition to estimate its influence. 

 The luxuriance of growth after a thund<;r&torm is well known to 

 be such as the mere quantity of rain which descends is not at all 

 sufficient to explain. 



C. The geological nature of the subsoil was the third point 

 mentioned. 



With respect to site, a gentle slope towards the south-west is the 

 most preferable, except in very thin, dry soils, and the lower the 

 land lies the more probability is there of accessions of inorganic 

 matter from above. A too rapid inclination causes the water to 

 run off too rapidly, often to the destruction of the soil. Under 

 these circumstances, however, the want of a pervious substratum 

 may be less felt ; and even where it is rocky, the shallowness of 

 mould, other circumstances being favourable, may be compen- 

 sated, and the cultivation of deep-rooting plants be possible. 

 Tlie mechanical composition of the substratum is also of great 

 import. Firm rocks, and difficult of decomposition, present an 

 absolute impediment to any penetration beyond the external soil. 

 Coarse masses of gravel or rubble, which have no power of im- 

 bibing the substances lixiviated from the soil and in turn of 

 restoring them when needful, are equally injurious. The most 

 profitable subsoil is afforded by alluvial or diluvial strata of sand, 

 clay, marl, &c., but of these the worst is sand, unless constantly 

 moist from lying at a sufficient depth. The great advantage of 

 these substrata is, that tillage may be carried to any depth, and 

 next in order come such strata as consist of stone, which is 

 loose and easily decomposed. Nor is its chemical nature of less 

 consequence. On this depends the richness of the soil in inor- 

 ganic constituents, and those subsoils which the more easily part 

 with these constituents are the most favourable, such as volcanic 

 ashes, lava, and basalt ; after which come feldspathic porphyry 

 and granite. Pure sand and lime, free from admixture of clay, 

 are the poorest. Those strata also which crop out and whose 

 fractured ends are exposed are better than those where the sur- 

 face follows exactly their dip. In fact, on the subsoil in great 

 measure depends the just degree of drainage, temperature, and 

 other circumstances, and in general the looser the soil itself is, 

 the firmer and more compact should be the substratum. 



II. There are, however, other circumstances which are more 

 or less under the power of the cultivator and on which success 

 immediately depends. He can form a soil where none is present, 



