In the discussion of ferns in the first chapter we found that on the back of some of their leaves, or occasionally on special leaves devoted to the purpose, were many small brownish or dark spots, arranged in rather definite fashion, and known as _sori_. (Figure 63.) Each sorus contains many minute bodies known as _spores_, not unlike very miniature seeds in general appearance, but quite unlike them in behavior and mode of life. No better idea of their size can be gleaned than to record the fact that in each sorus there may be about one hundred small, often short-stalked spore cases, known as _sporangia_, and that in each sporangium well over forty, and sometimes over sixty, spores will be crowded. A healthy specimen of many of our common ferns will bear about ten or a dozen leaves, each of which is divided into many divisions, and among these divisions of the leaf there may be at least fifty that bear from fifteen to twenty sori. It can be easily figured from this that a healthy plant of this fern may and usually does produce over forty-five million spores, each of which contains within it the opportunity of developing into a new plant. There is thus a prodigality in producing the means of renewal of life among ferns that far outstrips the production of seeds in even the most prolific of flowering plants. When the spores in the sporangium are mature and therefore ready for the next stage in their life history several things must happen. With somewhere about six thousand of them crowded together under each sorus, more room to develop is obviously the first consideration. This is provided for by the fact that when the spores are ripe the sporangia have the ability to throw them considerable distances; then of course the wind can carry them much farther. To be of any use they must fall upon damp ground, for some degree of moisture is absolutely necessary for what is about to happen to them. In nature countless millions never do fall in a favorable location, or, if they did, such an enormous production of fern spores would soon make the world exclusively a fern garden. The comparatively minute fraction of them that ever do find congenial surroundings, once they are expelled from the spore case, then begin a process that is not unlike the germination of a seed. For the spore must take in water from the soil, which by osmotic pressure finally bursts it open. From the burst spore a minute tube, known as the _protonema_, or literally first thread, begins to develop. It is, of course, of microscopic size, and yet near its base there is a branch tube formed, differing from it in structure and ultimately forming _rhizoids_, which are rootlike hairs. Both the protonema and the rhizoids begin growing, the first forming, usually flat on the ground, an often heart-shaped body having the characteristic green coloring matter of all plants. The rhizoids multiply and look not unlike roots. This young, still microscopic plant, grows apace, and may soon be