114 APPLICATION OF PRINCIPLES. 



■with food against the renewal of active vegetation : 

 hence, when trees are taken out of the earth in 

 autumn, and allowed to remain exposed to a dry air 

 all the winter, they either perish, or are greatly enfee- 

 bled. If, on . the other hand, the soil in which they 

 stand is filled with moisture, their system is dis- 

 tended with aqueous matter at a time when it cannot 

 be decomposed or thrown off, and the plant either 

 becomes naturally susceptible of the influence of cold 

 in rigorous climates (112), or is driven prematurely 

 into growth, when its new parts perish from the unfa- 

 vourable state of the air in which they are developed. 

 The most suitable condition of the soil, at the period 

 of vegetable rest, seem to be that in which no more 

 aqueous matter is contained than the results from the 

 capillary attraction of the particles. 



Nevertheless, there are exceptions to this, in the 

 case of aquatic and marsh plants, whose peculiar con- 

 stitution enables them to bear with impunity, during 

 winter, an immersion in water ; and in that of many 

 kinds of bulbs, which, during their season of rest, are 

 exposed to excessive heat. The latter plants are, 

 however, constructed in a peculiar manner ; their 

 roots are annual, and perish at the same time as the 

 leaves, when the absorbent organs are all lost, so that 

 the bulb cannot be supposed to require any supply 

 of moisture, inasmuch as it possesses no means of 

 taken it up, even if it existed in the soil. This will 

 be again adverted to in a future chapter. 



It is when plants are in a state of growth that 

 an abundant supply of moisture is required in the 



