The Fruiting of the Fern

Teacher's Story

HE fern, like the butterfly, seems to have several this-world incarnations; and perhaps the most wonderful of these is the spore. Shake the dust out of the ripened fern and each particle, although too small for the naked eye to see, has within it the possibilities of developing a mass of graceful ferns. Each spore has an outside hard layer, and within this an atom of fern-substance; but it cannot be developed unless it falls into some warm, damp place favorable for its growth; it may have to wait many years before chance gives it this favorable condition, but it is strong and patient and retains its vital power for years. There are cases known where spores grew after twenty years of waiting. But what does this microscopic atom grow into? It develops into a tiny heart-shaped, leaflike structure which botanists call the prothallium; this has on its lower side little roots which reach down into the soil for nourishment; and on its upper surface are two kinds of pockets, one round and the other long. In the round pockets are developed bodies which may be compared to the pollen; and in the long pockets, bodies which may be compared to the ovules of flowering plants. In the case of ferns, water is necessary to float the pollen from the round pockets to the ovules in the long pockets. From a germ thus fertilized in one of the long pockets, a little green fern starts to grow, although it may be several years before it becomes a plant strong enough to send up fronds with spore-dots on them.

Prothallium, greatly enlarged, showing the two kinds of pockets and the rootlets.

To study the structure of the spore requires the highest powers of the microscope; and even the prothallium in most species is very small, varying from the size of a pin-head to that of a small pea, and it is therefore quite difficult to find. I found some once on a mossy log that bridged a stream, and I was never so triumphant over any other outdoor achievement. They may be found in damp places, in greenhouses, but the teacher will be very fortunate who is able to show her pupils this stage of the fern. The prothallium is a stage of the fern to be compared to the flower and seed combined in the higher plants; but this is difficult for young minds to comprehend. I like to tell the children that the fern, like a butterfly, has several stages: Beginning with the spore-bearing fern, we next have the spores, next the prothallium stage, and then the young fern. While in the other case we have first the egg, then the caterpillar, then the chrysalis, and then the butterfly. Looking at the ripe fruit-dots on the lower side of the fern leaf, we can easily see with a lens a mass of tiny globules; each one of these is a spore-case, or sporangium (plural sporangia), and is fastened to the leaf by a stalk and has, almost encircling it, a jointed ring. (See figure on page 686).

Christmas fern is below the others.  Photo by Verne Morton.

When the spores are ripe, this ring straightens out and ruptures the globule, and out fly the spores. By scraping a little of the brown fuzz from a fruiting pinna of the Christmas fern upon a glass slide and placing a cover glass upon it, we find it very easy to examine through the microscope, and we are able thus to find the spore-cases in all stages, and to see the spores distinctly. The spore-cases may also be seen with a hand lens, the spores seeming then to be mere dust.

The life of a fern.  1. a, pinna bearing fruit;   b, a fruit-dot, enlarged, showing spore-cases pushing out around the edges of the indusium; c, spore-case, enlarged, showing how it discharges the spores. 2. Prothallium, enlarged. 3. Young fern growing from the prothallium.

The different ways the ferns blanket their spore-cases is a delightful study, and one which the pupils enjoy very much. All of our common ferns except the careless little polypody thus protect their spores. Whether this blanket be circular, or horseshoe-shaped, or oblong, or in the form of pocket or cup, depends upon the genus to which the fern belongs. The little protecting blanket-membrane is called the indusium,  and while its shape distinguishes the genus, the position in which it grows determines the species. I shall never forget my surprise and delight when, as a young girl, I visited the Philadelphia Centennial Exposition, and there in the great conservatories saw for the first time the tree-ferns of the tropics. One of these was labelled Dicksonia,  and mystified, I asked the privilege of examining the fronds for fruiting organs. When lo! the indusium proved to be a little cup, borne at the base of the tooth of the pinnule, exactly like that of our boulder fern, which is also a Dicksonia. I had a sudden feeling that I must have fern friends all over the world.

1. Fruiting pinnule of the boulder fern, enlarged. 2. Fruiting pinnules of spleenwort, enlarged.

Fruiting pinnules of evergreen wood fern.

Fruiting pinnules of the chain fern.

The children are always interested in the way the maidenhair folds over the tips of her scallops to protect her spore nursery; and while many of our ferns have their fertile fronds very similar in form to the sterile ones, yet there are many common ferns with fertile fronds that look so different from the others, that one would not think they were originally of the same pattern; but although their pinnules are changed into cups, or spore-pockets, of various shapes, if they be examined carefully they will be seen to have the same general structure and the same divisions however much contracted, as have the large sterile fronds. The Osmundas, which include the interrupted, the cinnamon and the flowering ferns, are especially good for this part of the lesson. The sensitive fern, so common in damp places in open fields, is also an excellent illustration of this method of fruiting. While studying the ferns, the teacher should lay stress upon the fact that they represent the earliest and simplest forms of plants, that they reached the zenith of their growth in the Carboniferous age, and that, to a large extent, our coal is composed of them. It is interesting to think that the exquisite and intricate leaf patterns of the ferns should belong to a primitive type. Often when I have watched the forming by the frost, of the exquisite fernlike pictures on the window-pane, I have wondered if, after all, the first expression of the Creator did not find form in the most exquisite grace and beauty; and if perchance the first fishes, so fierce and terrible, did not mark the introduction of Satan.

A sensitive fern, showing sterile and fertile fronds.

Method—July is the best time for this lesson, which is well adapted for summer schools or camping trips. However, if it is desired to use it as a school lesson, it should be begun in June, when the fruiting organs are green, and it may be finished in September after the spores are discharged. Begin with the Christmas fern, which ripens in June, and make the fruiting of this species a basis for comparison. Follow this with other wood ferns which bear fruit-dots on the back of the fronds. Then study the ferns which live in more open places, and which have fronds changed in form to bear the spores—like the sensitive, the ostrich, the royal and the flowering ferns. A study of the interrupted fern is a desirable preparation for the further study of those which have special fruiting fronds; the interrupted fern has, at about the middle of its frond, three pinnæ on each side, fitted for spore-bearing, the pinnules being changed into globular cups filled with spore-cases.

While not absolutely necessary, it is highly desirable that each member of the class should look at a fruit-dot of some fern through a three-quarters objective of a compound microscope, and then examine the spore-cases and the spores through a one-sixth objective. It must be remembered that this lesson is for advanced grades, and is a preparation for systematic scientific work. If a microscope is not available, the work may be done with a hand lens aided by pictures.

2. What conditions must the spores have in order to grow? What do they grow into? (See First Studies of Plant Life by Atkinson, p. 207).

3. Look at a ripe fruit-dot on the back of a fern leaf and see where the spores come from. Can you see with a lens many little, brown globules? Can you see that some of them are torn open? These are the spore-cases, called sporangia,  each globule being packed with spores. Can you see how the sporangia are fastened to the leaf by little stems?

4. Almost all our common wood ferns have the spore-cases protected by a thin membrane, the spore-blanket, when very young; this little membrane is called the indusium,  and it is of different shape in those ferns which do not have the same surname, or generic name. Study as many kinds of wood ferns as you can find. If the blanket, or indusium, is circular with a dent at the center where it is fastened to the leaf, and the spore-cases push out around the margin, it is a Christmas fern;  if horseshoe-shaped, it is one of the wood ferns;  if oblong, in rows on each side of the midrib, it is a chain fern;  but if oblong and at an angle to the midrib, it is a spleenwort;  if it is pocket-shaped and opening at one side, it is a bladder fern;  if it is cup-shaped, it is a boulder fern;  if it breaks open and lays back in star shape, it is a woodsia;  if the edge of the fern leaf is folded over all along its margin to protect the spore-cases, it is a bracken;  if the tips of the scallops of the leaf be delicately folded over to make a spore blanket, it is the maidenhair. 

5. If you know of swampy land where there are many tall brakes, look for a kind that has some of its pinnæ withered and brown. Examine these withered pinnæ, and you will see that they are not withered at all but are changed into little cups to hold spore-cases. This is the interrupted fern.  The flowering fern  has the pinnæ at its tip changed into cups for spore-cases. The cinnamon fern,  which grows in swampy places, has whole fronds which are cinnamon-colored and look withered, but which bear the spores. The ostrich fern,  which has fronds which look like magnificent ostrich feathers, has stiff, little stalks of fruiting fronds very unlike the magnificent sterile fronds. The sensitive fern,  which grows in damp meadows and along roadsides, also has contracted fruiting fronds. If you find any of these, compare carefully the fruiting with the sterile fronds, and note in each case the resemblance in branching and in pinnules and also the shape of the openings through which the spores are sifted out.

Diagram of the interrupted fern, showing the three pairs of fruiting pinnæ, and a part of one of these enlarged. This fern often has fronds four or five feet high.

6. Gather and press specimens of as many ferns in the fruiting stage as you can find, taking both sterile and fruiting fronds in those species which have this specialization.

7. Read in the geologies about the ferns which helped to make our coal beds.

Supplementary reading.—The Story of a Fern; First Studies of Plant Life, Atkinson; The Petrified Fern, M. L. B. Branch.

The bulb-bearing bladder fern.   This beautiful fern clothes the banks of damp ravines. It has, in addition to fruiting organs, buds on the stem, which take root.  Photo by Verne Morton.