DR. SCHLEIDEN'S THEOKY OF AGRICULTURE. 149 



east. Plants receive a far greater quantity of water from the 

 steam of the atmosphere than from what falls in the shape of 

 rain, which indeed is especially valuable as an indication of the 

 complete saturation of the air with water. 



The coast of Peru affords a most striking example. The soil 

 for eighteen degrees of latitude is a loose moveable sand. From 

 May, for six months, a thin veil of clouds covers the coasts from 

 nine or ten o'clock in the morning to thi-ee in the afternoon. 

 From the first appearance of the cloud the sand hills, as if by 

 enchantment, assume the features of a beautiful garden, and 

 support for half a year a vast quantity of beasts and horses, though 

 real rain scarcely falls once in five years. 



It has been supposed that rain brings down a certain quantity 

 of inorganic salts, which have even been estimated at 86 lbs. 

 per acre. Half, however, of the water sinks into the depths of 

 the earth to supply the springs, or runs off at once into the 

 streams ; and it carries off with it not only a great quantity of 

 soluble salts, but also of inorganic matter : and supposing that 

 so much as 86 lbs. are actually added every year to the soil, 

 and half the water be carried off as above mentioned, each lb. 

 of which contains only five grains of salts, 400 lbs. more of 

 soluble matter would be carried off annually than it receives. 



Annual plants, as soon as their seeds are well formed, seem 

 to be perfectly independent of any further supply of water ; 

 thus, for instance, the stem of cereals dies from below upwards 

 long before the grains have attained their full chemical evolution. 



The motion and pressure of the atmosphere, again, are of 

 great consequence as regards the process of exhalation and evo- 

 lution of gaseous matter, which covdd not take place in an 

 atmosphere completely saturated with moisture. 



One of the most important points, however, is temperature. 

 Though no tabular observations exist in reference to the subject, 

 it is quite certain that a particular degree of temperature is 

 necessary for the perfect development of each particular plant 

 during every portion of its existence ; and were we in possession 

 of good observations we should often be in a condition, from 

 antecedent circumstances of temperature in particular stages of 

 growth, to predict the final result. The vine, for instance, will 

 grow vigorously where, from want of sufficient temperature, 

 neither blossom nor fruit often come to perfection. It is obvious 

 that evils, when foreseen in good time, often cease to be evils. 



Light, again, has a very powerful influence on vegetation. 

 The luxuriance of many tropical climes, and the peculiarities of 

 vegetation in Alpine plants, depend greatly on the intensity of 

 the light. The development of the minute fungi which cause 

 mouldiness, on the contrary, is almost stopped by the same 



VOL. vir. M 



