The chief characters to remember about mosses are that they are very simple, but practically always green plants that have some differentiation into stem and leaf; that, while they have no vascular system, their structure and particularly the mode of reproduction suggests that they are not very distant from the ferns, and quite likely simple ancestors of them. These characters are of more importance than appears on the surface, as we shall presently see, for they mark mosses off from many other nonvascular flowerless plants which have quite different structure and altogether different mode of life.
If you will turn to the chapter on Plant Behavior and read particularly the sections on “Leaves as Factories for the Manufacture of Food” and “Borrowing from the Living and Robbing from the Dead,” you will see in the food habits of the plants there noted the great difference that exists between plants, like mosses and ferns, that have green coloring matter in them, and those we are about to mention that never do. The lack of this green coloring substance tells us at once that plants of this sort live only on the dead remains of other plants. In the case of these nonvascular flowerless plants there are certain modes of growth that, in some forms at least, are always associated with this scavenger-like food habit.
The common mushroom ([Figure 65]) is the best known of that large group of plants, called generally fungi, which produce no green coloring matter, have no leaves attached to a stem, and always live on decayed vegetable, or sometimes inhabit living animals, even man himself. The mushroom with its brownish stalk and buttonlike dome is familiar enough, but there are literally thousands of different kinds, a common sort forming “brackets” on the trunks of trees. While perhaps everyone would recognize these as plants, peculiar as they are in their often weird shapes and unusual as they nearly always are in their color, there are many minute kinds of fungi that scarcely anyone would even think of as a plant, and yet for better or worse they are incomparably the most powerful plants in the world. For upon these microscopic fungi man depends for many things. It is certain kinds of them that make the manufacture of cheese possible. They turn milk sour (pasteurizing milk is merely stopping their work), give to yeast its power of “raising” bread, all brewing depends upon them, every process of fermenting the juice of fruits for wine making or for whatever else, the decay of wood—all these processes and scores of others, whether for the good or evil of mankind, depend upon the work of these plants, any one of which is so small that a single individual must be magnified hundreds of times to detect it. Many of them are the “germs”—better called bacteria—that cause diseases like tuberculosis, cholera, typhoid, anthrax, and diphtheria. All surgeons wage incessant warfare against a host of them that attack wounds and form pus. They live in our intestines and have much to do with digestion, and unhappily with indigestion, so that we may be said to carry about with us a whole flora of them! Nearly all the diseases of plants, like the blight of potato and the rust on wheat, are caused by them. Some other kinds live in the soil, and many flowering plants depend absolutely for getting their food upon the work of these fungi. Unfortunately their minute size and consequently obscure mode of life demand technical skill and the use of the microscope to detect them, so we must leave them here, always keeping in mind that these smallest of all plants are charged with a power for good or evil; so far as man’s life is concerned, greater perhaps than all other plants.
While most fungi, particularly those familiar ones like mushrooms and puffballs, are inhabitants of the land, the remaining group of nonvascular flowerless plants are nearly all water plants. Most of the better known ones live in the sea, and as the wrack or tangle washed up on the shore we recognize them as seaweed. The algæ ([Figure 66]), which is a general name for such plants—and they live in the sea, in fresh water, and even on dry land—are, so far as structure is concerned, the simplest of all plants.
Those that are fastened to rocks are often beautifully colored, much branched, and many kinds bear small bladders that act as buoys. These coast seaweeds are generally of different colors, those nearest the surface being generally greenish, the deeper water kinds reddish or brown. None of these seaweeds are found at great depths, because the really deep parts of the ocean are almost, if not quite, dark. Seaweeds, and in fact all the algæ, have green coloring matter in them, even where this is masked by reds and browns, as is the case in some particularly showy kinds. As you will find in the section on “Leaves as Factories for the Making of Food,” no plant with green coloring matter can live in the dark. That is why seaweeds are not found in the great deeps of the sea, some of which are several miles below the shore line along the coasts, and are so cold and dark that neither plants nor animals can grow in them.
Those seaweeds that grow along the coast, and are uncovered by the retreating tides, are well known by everyone, but by far the greater number of algæ float without anchorage of any kind. One kind that has been torn from its anchorage occurs in such enormous quantities that off the coast of America it has formed literally a floating island composed entirely of dense mats of a species of seaweed. This place, known as the “Sargasso Sea” from the name of the seaweed forming it, was the terror of old mariners and Columbus’s ship was fouled in it for two weeks. The area occupied by the weed is several hundred miles long and wide, and while old sea yarns about ships being caught in it and never escaping are gross exaggerations, it is certainly one of the most curious of plant growths, due entirely to a nonvascular cryptogam.
Of those kinds that are never anchored the number is legion, and in addition to those forming the diatomaceous earth, already mentioned, there are many more. They form almost the only food of hosts of creatures of the sea, but because of their floating freely in the water, the consequent difficulty of collecting them, and their unusually minute size, little is likely to be known of them, except by the experts.
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.”