114 PLANT LIFE ON THE FARM. 



according to the nature of the manurial agent employed, 

 the ever varying character of the seasons, and the asso- 

 ciation or hostile competition of their neighbours. These 

 several conditions rarely, if indeed ever, act singly, but 

 almost always in combination. Circumstances are never 

 exactly twice alike ; a condition of absolute equilibrium is 

 never attained. The nearest approach to it has been reached 

 in the case of the unmanured plot on the one hand, and of 

 the very highly manured plots on the other, but these, like 

 the others, are influenced by climatal changes occurring 

 now at one stage of growth now at another. And even 

 when a comparative state of equilibrium is attained, very 

 slight causes, even such as may be roughly called acci- 

 dental, as the injuries inflicted by insects, or parasitic 

 fungi, suffice to disturb the balance and bring about a 

 different arrangement and proportion of species, and a 

 corresponding change in the development of individual 

 plants. 



As to the action of manures on the plants, it is com- 

 paratively rarely that they are employed in such quantities 

 as to be absolutely destructive or poisonous. In most 

 cases even when a particular manure is proved to be more 

 or less directly injurious to particular plants the indirect 

 harm accruing from the beneficial action of the substance 

 on some other plant or plants, growing in association with 

 them, is greater than the direct mischief. The manures 

 act very differently on different plants, and vary in their 

 action, even in the same species, according to the time and 

 stage of growth at which they are employed. Some 

 encourage the growth and development of their cellular 

 tissues, at the expense of the woody and fibrous constituents, 

 others favour the consolidation of the tissues, hasten the 



