476 INFLUENCE OF THE SOIL AND LIGHT. . 



Influence of ure, as affecting the well-being of plants, may be added the 

 the soil. chemical constitution of the soil upon which they grow. Lime- 

 plants can never be developed except on soils in which that earth abund- 

 antly occurs, and the same may be said of potash or soda plants, or, in 

 short, of any which demand some special mineral ingredient. Thus, for 

 instance, the salsolas and salicornias, which grow abundantly on the At- 

 lantic shores of France, and which require for their development the 

 saline ingredients of the sea, are nowhere to be seen throughout Central 

 Europe, though they reappear on the salt steppes of Russia, and abound 

 around the Caspian. We should scarcely expect that sea-weeds, into 

 the composition of which bromine and iodine abundantly enter, should 

 ever grow in waters from which these chemical elements are totally ab- 

 sent. Upon these principles, the vegetation of extensive tracts of coun- 

 try has undergone a change in an artificial way. Thus, for instance, in 

 Virginia and other Southern States, we may pass for miles in succession 

 through tracts in which the ancient forest-growths have been replaced 

 by the Pinus teeda, or old field pine. These are tracts from which the 

 potash salts have been removed, to a great extent, by the culture of to- 

 bacco. And of the indigenous trees, this pine requires the smallest pro- 

 portion of those salts. It therefore can flourish where -the others can 

 not exist. 



From what has been said in tjie last chapter, it may be inferred that 

 Influence of among the various conditions thus influencing the growth of 

 the sun's light. a pl an t, none are of greater importance than the amount of 

 light furnished to it. Through this agent the decomposition of carbonic 

 acid is effected, and the plant obtains from the air the carbon it requires, 

 out of which its solid structures are for the most part built. The rapid- 

 ity with which the reduction of the carbonic acid takes place depends 

 upon the brilliancy of the light, and the amount of carbon thus obtained 

 upon that condition and the time of exposure conjointly. The amount of 

 light received from the sun in any locality depends in a general way, as 

 does the heat, upon the latitude ; but in both cases a multitude of disturb- 

 ing agencies intervene. Variations of moisture control the supply of light 

 by permitting a translucency, or establishing its opposite, a cloudiness or 

 murkiness of the air. Other meteorological causes, as, for example, winds, 

 by condensing or removing moisture, act in like manner ; so also do as- 

 influence of tronomical conditions, especially by influencing the relative 

 the position of length of the day and night ; for, as we advance toward the 



the sun and , fo . J . b _ . . . _ , . 



length of the pole, the summer sun is above the horizon longer and longer. 

 da y- I n Northern Europe, during the month of June, he never sets, 



but remains all night, if night it can be called, above the horizon ; and, 

 as Berzelius well remarks, "Under the influence of this midnight sun of 

 the North, the life of plants runs through the same cycle of change in 



