Other algæ are always found in fresh water and form the scum found on stagnant pools. Individuals of any of these are so minute that, while under the microscope they are of the greatest beauty, their structure must remain for most of us a sealed book. SUMMARY OF WHAT PLANTS ARE We have now traced, in only the briefest fashion, the outlines of what plants are, reversing the order of nature in beginning with those most complex but best known, the flowering plants. As we shall see later, these are the climax of prodigal nature and are to be considered the end rather than the beginning of plant life on the earth. Then, and still more briefly, have we stopped to see those less known plants that produce no flowers, such as the ferns, mosses, fungi, and finally the seaweeds or algæ. These are all to be considered as the ancestors of flowering plants, the ferns the nearest to them and the algæ probably the most distant relatives. The development of plants from the minutest alga up to our most gorgeous flowering plant, is an infinitely slow and painful process. With many mistakes, with its pathway strewn with the wreckage of forlorn hopes and false starts, it is incomparably the most dramatic story in the plant world. Some of its details will be told in the chapter on the “History of the Plant Kingdom.” Nor can we leave the discussion of what plants are without some mention of the thing that really makes up their structure, whether it be a microscopic bacterial organism or the Big Tree of California. For the unit of all animal and plant life is the _cell_. In its simplest form it is merely a minute sac with a definite wall and inside the wall is a substance known as _protoplasm_, literally _protos_, first, and _plasma_, thing formed. It is protoplasm that forms the living tissue of all plants and animals; it is life itself. No one has ever succeeded in making any, notwithstanding that many learned men have tried for years. Its inclosure in the cell wall, its power of self-division and consequent multiplication of the units, make up those first things about which most of us can never know much, but the end of which we recognize in the beauty of plant life all about us. For only under the highest powers of the microscope may cells be seen and studied. Just as bankers reckon mills as a definite unit of a cent, and yet none of them has ever seen a mill, so we must think of cells as the definite unit of all living things, although most of us will never see a cell. But, unlike the mill, cells may be seen by those equipped to see them, and this study, the development and grouping of them to form all the varied objects that inhabit the plant world, is known as _histology_. It is literally the internal history of plants and animals, and lies quite outside the scope of this book. What we must never forget is that whatever knowledge we have gained, either from the foregoing account of what plants are, or from our observation of them, is, after all, only a