DIMORPHISM OF FERNS.
=563.= In comparing the different members of the leaf series there
are often striking illustrations of the transition from one form to
another, as we have noted in the case of the trillium flower. This
occurs in many other flowers, and in some, as in the water-lily, these
transformations are always present, here showing a transition from the
petals to the stamens. In the bud-scales of many plants, as in the
butternut, walnut, currant, etc., there are striking gradations between
the form of the simple bud-scales and the form of the leaf. Some of the
most interesting of these transformations are found in the dimorphic
ferns.
=564. Dimorphism in the leaves of ferns.=—In the common polypody
fern, the maidenhair, and in many other ferns, all the leaves are of
the same form. That is, there is no difference between the fertile leaf
and the sterile leaf. On the other hand, in the case of the Christmas
fern we have seen that the fertile leaves are slightly different from
the sterile leaves, the former having shorter pinnæ on the upper half
of the leaf. The fertile pinnæ are here the shorter ones, and perform
but little of the function of carbon conversion. This function is
chiefly performed by the sterile leaves and by the sterile portions of
the fertile leaves. This is a short step toward the division of labor
between the two kinds of leaves, one performing chiefly the labor of
carbon conversion, the other chiefly the labor of bearing the fruit.
[Illustration: Fig. 316. Sensitive fern; normal condition of vegetative
leaves and sporophylls.]
=565. The sensitive fern.=—This division of labor is carried to
an extreme extent in the case of some ferns. Some of our native ferns
are examples of this interesting relation between the leaves like
the common sensitive fern (Onoclea sensibilis) and the ostrich fern
(O. struthiopteris) and the cinnamon-fern (Osmunda cinnamomea). The
sensitive fern is here shown in fig. 316. The sterile leaves are large,
broadly expanded, and pinnate, the pinnæ being quite large. The fertile
leaves are shown also in the figure, and at first one would not take
them for leaves at all. But if we examine them carefully we see that
the general plan of the leaf is the same: the two rows of pinnæ which
are here much shorter than in the sterile leaf, and the pinnules, or
smaller divisions of the pinnæ, are inrolled into little spherical
masses which lie close on the side of the pinnæ. If we unroll one
of these pinnules we find that there are several fruit dots within
protected by this roll. In fact when the spores are mature these
pinnules open somewhat, so that the spores may be disseminated.
[Illustration: Fig. 317. Sensitive fern; one fertile leaf nearly
changed to vegetative leaf.]
There is very little green color in these fertile leaves, and what
green surface there is is very small compared with that of the broad
expanse of the sterile leaves. So here there is practically a complete
division of labor between these two kinds of leaves, the general plan
of which is the same, and we recognize each as being a leaf.
[Illustration: Fig. 318. Sensitive fern, showing one vegetative leaf
and two sporophylls completely transformed.]
=566. Transformation of the fertile leaves of onoclea to sterile
ones.=—It is not a very rare thing to find plants of the sensitive
fern which show intermediate conditions of the sterile and the
fertile leaf. A number of years ago it was thought by some that this
represented a different species, but now it is known that these
intermediate forms are partly transformed fertile leaves. It is a
very easy matter in the case of the sensitive fern to produce these
transformations by experiment. If one in the spring, when the sterile
leaves attain a height of 12 to 16 _cm_ (8-10 inches), cuts them away,
and again when they have a second time reached the same height, some
of the fruiting leaves which develop later will be transformed. A few
years ago I cut off the sterile leaves from quite a large patch of
the sensitive fern, once in May, and again in June. In July, when
the fertile leaves were appearing above the ground, many of them
were changed partly or completely into sterile leaves. In all some
thirty plants showed these transformations, so that every conceivable
gradation was obtained between the two kinds of leaves.
[Illustration: Fig. 319. Normal and transformed sporophyll of sensitive
fern.]
=567.= It is quite interesting to note the form of these changed
leaves carefully, to see how this change has affected the pinnæ and the
sporangia. We note that the tip of the leaf as well as the tips of all
the pinnæ are more expanded than the basal portions of the same.
This is due to the fact that the tip of the leaf develops later
than the basal portions. At the time the stimulus to the change in
the development of the fertile leaves reached them they were partly
formed, that is the basal parts of the fertile leaves were more or less
developed and fixed and could not change. Those portions of the leaf,
however, which were not yet completely formed, under this stimulus, or
through correlation of growth, are incited to vegetative growth, and
expand more or less completely into vegetative leaves.
=568. The sporangia decrease as the fertile leaf expands.=—If
we now examine the sporangia on the successive pinnæ of a partly
transformed leaf we find that in case the lower pinnæ are not changed
at all, the sporangia are normal. But as we pass to the pinnæ which
show increasing changes, that is those which are more and more
expanded, we see that the number of sporangia decrease, and many of
them are sterile, that is they bear no spores. Farther up there are
only rudiments of sporangia, until on the more expanded pinnæ sporangia
are no longer formed, but one may still see traces of the indusium.
On some of the changed leaves the only evidences that the leaf began
once to form a fertile leaf are the traces of these indusia. In some of
these cases the transformed leaf was even larger than the sterile leaf.
=569. The ostrich fern.=—Similar changes were also produced in
the case of the ostrich fern, and in fig. 319 is shown at the left a
normal fertile leaf, then one partly changed, and at the right one
completely transformed.
=570. Dimorphism in tropical ferns.=—Very interesting forms
of dimorphism are seen in some of the tropical ferns. One of these
is often seen growing in plant conservatories, and is known as the
staghorn fern (Platycerium alcicorne). This in nature grows attached to
the trunks of quite large trees at considerable elevations on the tree,
sometimes surrounding the tree with a massive growth. One kind of leaf,
which may be either fertile or sterile, is narrow, and branched in a
peculiar manner, so that it resembles somewhat the branching of the
horn of a stag. Below these are other leaves which are different in
form and sterile. These leaves are broad and hug closely around the
roots and bases of the other leaves. Here they serve to catch and
retain moisture, and they also catch leaves and other vegetable matter
which falls from the trees. In this position the leaves decay and then
serve as food for the fern.
[Illustration: Fig. 320. Ostrich fern, showing one normal sporophyll,
one partly transformed, and one completely transformed.]