PHENOGAMS AND CRYPTOGAMS
[Illustration: FIG. 254.—CHRISTMAS FERN.—Dryopteris acrostichoides;
known also as Aspidium. ]
The plants thus far studied produce flowers; and the flowers produce
seeds by means of which the plant is propagated. There are other
plants, however, that produce no seeds, and these plants (including
bacteria) are probably more numerous than the seed-bearing plants.
These plants propagate by means of =spores=, _which are generative
cells, usually simple, containing no embryo_. These spores are very
small, and sometimes are not visible to the naked eye.
[Illustration: FIG. 255.—FRUITING FROND OF CHRISTMAS FERN.
Sori at _a_. One sorus with its indusium at _b_.]
Prominent among the spore-propagated plants are _ferns_. The common
_Christmas fern_ (so called because it remains green during winter)
is shown in Fig. 254. The plant has no trunk. The leaves spring
directly from the ground. The leaves of ferns are called =fronds=.
They vary in shape, as other leaves do. Some of the fronds in Fig.
254 are seen to be narrower at the top. If these are examined more
closely (Fig. 255), it will be seen that the leaflets are contracted
and are densely covered beneath with brown bodies. These bodies are
collections of =sporangia= or =spore-cases=.
[Illustration: FIG. 256.—COMMON POLYPODE FERN. _Polypodium vulgare_.]
[Illustration: FIG. 257.—SORI AND SPORANGIUM OF POLYPODE. A chain
of cells lies along the top of the sporangium, which springs back
elastically on drying, thus disseminating the spores. ]
[Illustration: FIG. 258.—THE BRAKE FRUITS UNDERNEATH THE REVOLUTE
EDGES OF THE LEAF.]
[Illustration: FIG. 259.—FRUITING PINNULES OF MAIDENHAIR FERN.]
The sporangia are collected into little groups, known as =sori=
(singular, sorus) or =fruit-dots=. Each sorus is covered with a thin
scale or shield, known as an =indusium=. This indusium separates from
the frond at its edges, and the sporangia are exposed. Not all ferns
have indusia. The polypode (Figs. 256, 257) does not; the sori are
naked. In the brake (Fig. 258) and maidenhair (Fig. 259) the edge
of the frond turns over and forms an indusium. The nephrolepis or
sword fern of greenhouses is allied to the polypode. The sori are
in a single row on either side the midrib (Fig. 260). The indusium
is circular or kidney-shaped and open at one edge or finally all
around. The Boston fern, Washington fern, Pierson fern, and others,
are horticultural forms of the common sword fern. In some ferns (Fig.
261) an entire frond becomes contracted to cover the sporangia.
[Illustration: FIG. 260.—PART OF FROND OF SWORD FERN. To the pupil:
Is this illustration right side up? ]
The sporangium or spore-case of a fern is a more or less globular
body and usually with a stalk (Fig. 257). _It contains the spores.
When ripe it bursts and the spores are set free._
[Illustration: FIG. 261.—FERTILE AND STERILE FRONDS OF THE SENSITIVE
FERN.]
In a moist, warm place _the spores germinate_. They produce a small,
flat, thin, green, more or less heart-shaped membrane (Fig. 262).
This is the =prothallus=. Sometimes the prothallus is an inch or
more across, but oftener it is less than a ten cent piece in size.
Although easily seen, it is commonly unknown except to botanists.
Prothalli may often be found in greenhouses where ferns are grown.
Look on the moist stone or brick walls, or on the firm soil of
undisturbed pots and beds; or spores may be sown in a damp, warm
place.
[Illustration: FIG. 262.—PROTHALLUS OF A FERN. Enlarged:
Archegonia at _a_; antheridia at _b_.]
On the under side of the prothallus two kinds of organs are
borne. These are the =archegonium= (containing egg-cells) and the
=antheridium= (containing sperm-cells). These organs are minute
specialized parts of the prothallus. Their positions on a particular
prothallus are shown at _a_ and _b_ in Fig. 262, but in some ferns
they are on separate prothalli (plant diœcious). _The sperm-cells
escape from the antheridium and in the water that collects on the
prothallus are carried to the archegonium, where fertilization
of the egg takes place._ From the fertilized egg-cell a plant
grows, becoming a “fern.” In most cases the prothallus soon dies.
The prothallus is the =gametophyte= (from Greek, signifying the
fertilized plant).
The fern plant, arising from the fertilized egg in the archegonium,
becomes a perennial plant, each year producing spores from its fronds
(called the =sporophyte=); but these spores—which are merely detached
special kinds of cells—produce the prothallic phase of the fern
plant, from which new individuals arise. _A fern is fertilized but
once in its lifetime._ The “fern” bears the spore, the spore gives
rise to the prothallus, and the egg-cell of the prothallus (when
fertilized) gives rise to the fern.
A similar =alternation of generations= runs all through the vegetable
kingdom, although there are some groups of plants in which it is very
obscure or apparently wanting. It is very marked in ferns and mosses.
In algæ (including the seaweeds) the gametophyte is the “plant,”
as the non-botanist knows it, and the sporophyte is inconspicuous.
_There is a general tendency, in the evolution of the vegetable
kingdom, for the gametophyte to lose its relative importance and for
the sporophyte to become larger and more highly developed._ In the
seed-bearing plants the sporophyte generation is the only one seen by
the non-botanist. The gametophyte stage is of short duration and the
parts are small; it is confined to the time of fertilization.
The sporophyte of seed-plants, or the “plant” as we know it, produces
two kinds of spores—one kind becoming =pollen-grains= and the other
kind =embryo-sacs=. The pollen-spores are borne in sporangia,
which are united into what are called =anthers=. The embryo-sac,
which contains the egg-cell, is borne in a sporangium known as an
=ovule=. _A gametophytic stage is present in both pollen and embryo
sac: fertilization takes place, and a sporophyte arises. Soon this
sporophyte becomes dormant, and is then known as an_ =embryo=. The
embryo is packed away within tight-fitting coats, and the entire body
is the =seed=. When the conditions are right the seed grows, and
the sporophyte grows into herb, bush, or tree. The utility of the
alternation of generations is not understood.
The spores of ferns are borne on leaves; the spores of seed-bearing
plants are also borne amongst a mass of specially developed
conspicuous leaves known as =flowers=; therefore these plants
have been known as the =flowering plants=. Some of the leaves are
developed as envelopes (calyx, corolla), and others as spore-bearing
parts, or =sporophylls= (stamens, pistils). But the spores of the
lower plants, as of ferns and mosses, may also be borne in specially
developed foliage, so that the line of demarcation between flowering
plants and flowerless plants is not so definite as was once supposed.
The one definite distinction between these two classes of plants is
the fact that _one class produces seeds and the other does not_. The
seed-plants are now often called =spermaphytes=, but there is no
single coordinate term to set off those which do not bear seeds. It
is quite as well, for popular purposes, to use the terms =phenogams=
for the seed-bearing plants and =cryptogams= for the others. These
terms have been objected to in recent years because their etymology
does not express literal facts (_phenogam_ signifying “showy
flowers,” and _cryptogam_ “hidden flowers”), but the terms represent
distinct ideas in classification. The cryptogams include three great
series of plants—the =Thallophytes= or algæ, lichens, and fungi; the
=Bryophytes= or moss-like plants; the =Pteridophytes= or fernlike
plants.
[Illustration: FIG. 263.—DIAGRAM TO EXPLAIN THE TERMINOLOGY OF THE
FROND. ]
SUGGESTIONS.—=186.= _The parts of a fern leaf._ The primary
complete divisions of a frond are called pinnæ, no matter whether
the frond is pinnate or not. In ferns the word “pinna” is used
in essentially the same way that leaflet is in the once-compound
leaves of other plants. The secondary leaflets are called pinnules,
and in thrice, or more, compound fronds, the last complete parts
or leaflets are ultimate pinnules. The diagram (Fig. 263) will aid
in making the subject clear. If the frond were not divided to the
midrib, it would be simple, but this diagram represents a compound
frond. The general outline of the frond, as bounded by the dotted
line, is ovate. The stipe is very short. The midrib of a compound
frond is known as the rachis. In a decompound frond, this main
rachis is called the primary rachis. Segments (not divided to the
rachis) are seen at the tip, and down to _h_ on one side and to _m_
on the other. Pinnæ are shown at _i_, _k_, _l_, _o_, _n_. The pinna
_o_ is entire; _n_ is crenate-dentate; _i_ is sinuate or wavy, with
an auricle at the base; _k_ and _l_ are compound. The pinna _k_ has
twelve entire pinnules. (Is there ever an even number of pinnules
on any pinna?) Pinna _l_ has nine compound pinnules, each bearing
several entire ultimate pinnules. _The spores._—=187.= Lay a mature
fruiting frond of any fern on white paper, top side up, and allow
it to remain in a dry, warm place. The spores will discharge on the
paper. =188.= Lay the full-grown (but not dry) cap of a mushroom or
toadstool bottom down on a sheet of clean paper, under a ventilated
box in a warm, dry place. A day later raise the cap.