PHENOGAMS AND CRYPTOGAMS
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. 254.--CHRISTMAS FERN.--Dryopteris acrostichoides;
known also as Aspidium.]
[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?]
[Illustration: FIG. 261.--FERTILE AND STERILE FRONDS OF THE SENSITIVE
FERN.]
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._
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 dime 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 coördinate 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 mosslike 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.