You must have noticed little dark brown spots on the backs of some fern-leaves. It is in them that you must look for the beginning of the new fern plants. The little patches are at first hidden by green coverings, but when they are ripe these bend back, and expose the little brown clusters within. If you look at one of these ripe patches with a magnifying-glass, you may be able to see a number of little roundish boxes on stalks. Each of these contains a number of tiny “spores” (which are ~single cells~ with the power to grow), and when the spore-cases are ripe they open and shoot out the spores, as you may perhaps see if you look closely at a ripe patch when it is taken into warm, dry air. [Illustration: Fig. 128. A small piece of Fern leaf showing the patches of spore-cases on the under side.] These brown patches are not at all like flowers, but in some way they do the work of flowers, for they give rise to cells which can carry on the life of the fern to a second generation. The way in which they do it, however, is totally different from that of the seed, and is quite the most special character of the ferns and their relatives. The spores grow slowly when they come on to moist earth, but as their development takes a long time, you had better get some from a gardener which have already grown. As the spore grows, the one cell composing it divides and divides again, until there is formed a little filmy heart-shaped green structure called a ~prothallium~ (~see~ fig. 129), which is not in the least like a fern plant, for it is not more than a quarter of an inch across. It has no stem or leaves, and is only a thin layer of green cells, with a few root-hairs on the under side. Two of the cells formed on this little structure then unite and begin to grow while still attached to it, and finally they grow into the form of a very small, simple fern plant (~see~ fig. 129). So that between the old fern plant and the little fern “sporeling” (for we cannot call it a seedling) we find a whole new structure, the prothallium, which is quite different from the usual fern plant. This curious alternation of fern,--prothallium,--fern, and then again prothallium, is what we call “alternation of generations,” and is very characteristic indeed of the fern tribe. [Illustration: Fig. 129. A Prothallium (~p~), with a young Fern (~f~) growing out from it. (Magnified).] Some ferns take a short cut, and bear little ones directly on their leaves without any prothallium. You see this in the “Hundreds and Thousands” fern, where the old plant is sometimes covered over with little ones, which will grow if they are taken off and planted carefully.