RELATIONSHIPS OF METASPERMAE. 583 



presence of the harmful foreign weed is heralded, and measures 

 are taken to prevent its securing a foothold, for it is understood 

 that if it be a plant of robust habit it will conquer for itself an 

 abiding place at the expense of other weaker plants with which 

 it may come in contact during its struggle for existence. Every 

 individual plant must make its way in the world. It must either 

 win new territory, maintain w^hat it has already won, or cede 

 its place of abode and growth to some plant better fitted to 

 cope with the conditions peculiar to that particular spot. It 

 thus happens that the flora of any region — that is to say the 

 plant society of the region — is in the same condition of mutual 

 interdependence and mutual competition that we discover in 

 human society. Complex inter-relations of individual with in- 

 dividual, species with species, formation with formation arise 

 and the plant- population of any area so far from being stable 

 in its composition is in a continual state of battle for soil, light, 

 moisture, heat and useful alliances, both in the physical and 

 biological sense of the word. Thus, in a forest, the pine- 

 trees compete with each other for light, each taller one than 

 the rest gaining a distinct advantage; hard wood timber an- 

 tagonises the coniferous and along the forest skirmish-line will 

 be found slowly working its way up the streams, gradually 

 isolating the coniferous trees into separate groves, ready 

 at the first sign of misfortune or weakness in the opposing 

 species to seize and occupy its territory. Again forest and 

 prairie — the two most notable plant formations of the Minne- 

 sota valley — each tenanted by hundreds of species characteris 

 tic if not peculiar — ^carry on a silent warfare with each other 

 and as the chance of battle swings in the favor of the one, the 

 other is imperceptibly but surely driven back. 



It happens then, to return to the illustration, that we find 

 plants organised much as is human society. The individuals 

 of each species compete with each other for favorable habi- 

 tats and for the optimum of growth-materials and energising 

 forces. Each species competes with those around it and in 

 this competion the individuals might be said to stand shoulder 

 to shoulder against the common foe, as may be seen in the 

 united efforts of a human tribe or nation against some warring 

 body. And again groups of species, having perhaps a common 

 line of movement or a common need to be supplied, band them- 

 selves together and find arrayed against them other united 

 groups of species competing for the same necessity or striving 

 to move in the opposite direction. 



