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. Itmust either 
win new territory, maintain what 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 
inits 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. 
