90 PLANT LIEE OJS THE FAKM. 



But -apart from this external conflict with the elements, 

 plants are always more or less in a state of internecine 

 war. Plants of different kinds growing wild in a state of 

 nature may contend one with another for root-hold, soil- 

 food, and for space to expose their foliage to the sun. 

 Under such circumstances, if there is enough for all, it 

 may be that the severity of the struggle may be slight, 

 owing to the different requirements of the different 

 plants, but even then the stronger of the two will eventu- 

 ally prevail. A farmer, however, would hardly call the 

 preponderance of weeds an instance of the survival of 

 the fittest. From his point of view it would certainly 

 not be so, however true it might be in wild nature. 

 Plants of the same kind growing gregariously, like heaths 

 on a moor, have the same requirements, and these are 

 supplied in about equal proportions to all the individual 

 plants. The result is that while the weak ones are 

 crowded out, the survivors are all pretty much on an 

 equality ; but once the balance is destroyed, then that 

 which is the stronger, or the one best adapted to the cir- 

 cumstances under which it is placed, will survive. 



In cultivation we have illustrations of mixed and of 

 gregarious vegetation in the sense above employed, as 

 well as of alternate vegetation as in the case of "rotation." 

 In the case of the cereals, of turnips, of potatoes, and 

 others, we have instances of gregarious vegetation 

 induced, indeed, by the will of the cultivator. His 

 object is to secure the most profitable development of 

 one particular kind of plant, wheat, barley, oats, or what 

 not. To compass this end he grows them together, 

 takes means by appropriate tillage, and by the removal 

 of competing weeds, to enhance the conditions most 

 favorable to their growth and to minimize the effects of 

 those that are injurious. The warfare here is external 

 as regards "weeds," it is internecine between individual 

 plants of the same species and having the same require- 



