Thus all co-operate in sunlight-catching. But, when a number of different plants are competing together to catch the light on one square yard of ground, their leaves try to overreach and get beyond their neighbours. On such a square yard of ground, it is just the competition amongst the plants, that makes it certain that every gleam of light is used by one or other of them. Every one of all those plants of itself alters the slope of its leaves and turns its stems so as to get as much light as possible. This light, as we have seen, is taken in by the plant. It is used to make the gas, carbonic acid,[2] unite with water: when these are made to join together, they form sugar; if the sugar is burnt the heat and light appear again. [2] The gas Carbonic acid consists of one part of Carbon and two of Oxygen. It is invisible, just as are the gaseous states of many liquids and solids. Water-vapour is not visible, though water (liquid) and ice can of course be seen. Starch, sugar, cell wall substance, etc., all contain Carbon, Oxygen, and Hydrogen. Vegetable fat is not well understood, but starch helps to form it. By changing the amount and arrangement of the molecules in sugar, starch or vegetable fats, and many other substances can be formed. But it is the sunlight that makes all this possible. Thus the sun not merely supplies the motive power for all animal and vegetable activity but, by its influence, flowers, leaves, and stems move and turn in such ways that they are in the most convenient position to intercept its light. The sunlight, though all-important in the life of most plants, kills many kinds of bacteria and bacilli which love the darkness. The well-known radium rays are also destructive to bacteria, and hinder the growth of certain fungi (Becquerel's rays have a similar effect). The X-rays are not so well understood, but one can close the leaflets of the Sensitive Plant by means of them. Carbonic acid gas forms but a small proportion of the atmosphere which surrounds a growing plant. Yet there is no lack of it, for when the leaf is at work forming sugar the particles of gas are rushing into the leaf, and other particles come from elsewhere to take their place. Every fire and every breath given off by an animal yields up carbonic acid, so that it is constantly in circulation.