70 BIOLOGY OF PLANTS. 



to the nature of the plant; with food, in kind and quantity ^ 

 suited to the age and wants of each species, and with proper 

 tillage. Especially must he exterminate those natural foes, 

 which make their appearance during the summer months, and 

 which may easily be overcome if attacked before they have 

 obtained a firm footing on the soil ; but let him remember, 

 that they possess a wonderful fecundity, and, like a certain 

 animal, have many lives, they must, therefore, be made to die 

 many deaths, before they can be completely exterminated. 



The importance of supplying the conditions, for exciting 

 the vital principle of seeds, and for its most perfect action 

 during the growth of plants, may be illustrated by reference 

 to the mechanic arts. In all these arts there is some agent, 

 natural or artificial, employed as a moving power. 



1. In locomotives and steamboats, the main spring of the 

 whole movement is the expansive force of steam, when sub. 

 jected to a high temperature. But steam has no power unless 

 supplied with the appropriate conditions. If made in the open 

 air it will not move a steamboat, though it may a feather. If 

 simply confined in a boiler, it will manifest its power in no way 

 unless it be to break from its confinement, and gain its free- 

 dom. A complicated apparatus must be supplied, the result 

 of intense study, and multiplied experiment, before its power 

 is available for any useful purpose ; and finally, various other 

 conditions must be added, before it will propel us across the 

 land or the ocean. 



2. In most cotton and woollen factories the moving force is 

 water. This power also requires several conditions, before 

 it can be usefully applied. The force of running water, 

 though often very great, will not manufacture cotton and wool- 

 len cloths. If it is arrested in its progress to the ocean, its 

 force will only be exerted upon the sides of the dam. Fac- 

 tories must be erected, wheels of the proper size and form 

 must be constructed. There must be added a complicated 

 apparatus of cards, spindles and looms, and after all this, cot- 



