6 FLOWERING PLANTS 
ways—by developing a thick covering of hairs, or a layer of 
wax, or a very thick skin. In the next chapter the response 
of the plant to its surroundings, as far as its external form is. 
concerned, is discussed, whilst Part I. treats of the adaptability 
of the plant to its environment in the minute structure of its 
vegetative organs. 
Not only is there a constant struggle for life going on 
- among plants owing to the variability of external conditions, 
but far more to the competition amongst themselves. That 
this must be so is evident when the number of the offspring 
of a single plant is considered. The number of seedlings 
produced by a single sycamore or an oak in its lifetime is 
immense ; it can hardly be less than millions when the length 
of life of these trees is taken into account; but most of these 
seedlings will die, even if left alone by man, owing to the 
want of light and food, due to the want of space. No plant 
can increase in numbers unless it can find a suitable place for 
its seeds to grow in, or unless it can overcome weaker plants. 
In this severe competition, it is the plant which shows itself 
most capable of adaptability to its surroundings that survives. 
This brings us to the study of the plants that grow together, 
forming Plant Associations, in some respects the most inter- 
esting part of Ecology. 
Plant Asso. A mountain, a moor, a valley, a wood, a marsh, 
clations. even a hedgerow or a stone wall, each has its 
characteristic vegetation, which may differ from that of the 
adjacent moor or valley, etc. In some districts it is easy to 
see that one species dominates all others: thus gorse is the 
characteristic species of many commons; in other places 
heather is apparently struggling with gorse, and where there 
is a deficiency of peat will probably be conquered. Again, in 
some forests the beech is the prevalent tree, in others the oak, 
whilst in some cases a struggle is going on between the two. 
Warming states that in Denmark the oak has been replaced 
by the beech, but in West Jutland it is able to hold its own 
against the beech. Side by side with these ‘‘ dominant ” 
species, as they are called, there exist other species occupying 
