1226 
MESSRS. J. B. LAWES, J. H. GILBERT, AND M. T. MASTERS, 
Not only may the varying mechanical or chemical nature of the soil influence the 
issue of the struggle, but also the varied requirements of plants as to water, light, and 
heat. Naegeli gives some examples of this. When Primula officinalis and P. elatior 
occur together in the same district they are sometimes sharply separated from one 
another; P. officinalis preferring the dry, P. elatior the damp spots. Each is most 
vigorous in its own special habitat, and may expel the other. But if only one species 
occurs in the district it is not so exacting; P. officinalis may then exist in damp, 
P. elatior in dry situations. 
The influence, direct or indirect, that may he exerted by insects and other forms of 
animal life on the vegetation of the plots is too large a subject to be more than inciden¬ 
tally mentioned in this place. That influence may be detrimental or it may be favour¬ 
able to plant growth or plant fertilisation ; and its importance, from the latter point of 
view especially, has been more fully recognised since the publication of Mr. Darwin’s 
observations and inferences ; but the general question was not overlooked by Linnaeus 
and the older naturalists, who recognised the interdependence of insects and plants, 
and pointed out how the equilibrium between the several classes was maintained.* 
The foregoing illustrations will convey some idea of the manifold conditions affecting 
the competition between neighbouring plants. Plants growing in association are sub¬ 
jected to the competition of their immediate allies having the same requirements as to 
food, moisture, space, light, &c., or to the more active antagonism of less nearly related 
plants, which, though making somewhat different demands on the environment, yet 
overcome their neighbours by their superior hardiness or force of growth. Or the 
equilibrium may be disturbed by causes external to the plants themselves. On the 
other hand, plants may grow together in comparatively peaceful association, because 
their requirements are different, or because the conditions are so unfavourable for the 
luxuriance of any in particular that many exist on somewhat equal terms of limited 
growth. 
The competition may be chiefly among the roots which form a network more or 
less dense and to a greater or less depth, according to the particular plant. The 
roots extend or ramify at different angles, and in different directions and to varying 
depths, according to the mechanical, chemical, and hygroscopic qualities of the soil. 
Just beneath the surface are the rootstocks, firm and more or less woody, jostling their 
neighbours, and pushing them aside, or emitting slender creeping stolons to twine their 
devious course among other plants. Or the competition may be chiefly above-ground. 
The dense tufts of some grasses occupy the surface, and prevent the growth of neigh¬ 
bouring species. In other cases, the creeping stems, the offsets, the runners—a hundred 
different forms and adaptations, provided for the benefit of the individual plant, as 
weapons of offence or defence, or as store houses for nutriment—contribute to the 
result. So again, also, is it with the form, the arrangement, the development of leaves, 
and also of the flowers and seeds. Indeed, the whole plant, in all its parts, and in all 
* See Amcen. AcacL,vi., passim, cited in Lyell’s ‘Principles of Geology,’vol. ii., ed. 10, p. 436. 
