COMPARISON OF FERNS AND THEIR RELATIVES.
=596. Comparison of selaginella and isoetes with the ferns.=—On
comparing selaginella and isoetes with the ferns, we see that the
sporophyte is, as in the ferns, the prominent part of the plant. It
possesses root, stem, and leaves. While these plants are not so large
in size as some of the ferns, still we see that there has been a great
advance in the sporophyte of selaginella and isoetes upon what exists
in the ferns. There is a division of labor between the sporophylls,
in which some of them bear microsporangia with microspores, and some
bear macrosporangia with only macrospores. In the ferns and horsetails
there is only one kind of sporophyll, sporangium, and spore in a
species. By this division of labor, or differentiation, between the
sporophylls, one kind of spore, the microspore, is compelled to form
a male prothallium, while the other kind of spore, the macrospore, is
compelled to form a female prothallium. This represents a progression
of the sporophyte of a very important nature.
=597.= On comparing the gametophyte of selaginella and isoetes
with that of the ferns, we see that there has been a still farther
retrogression in size from that which we found in the independent and
large gametophyte of the liverworts and mosses. In the ferns, while it
is reduced, it still forms rhizoids, and leads an independent life,
absorbing its own nutrient materials, and assimilating carbon. In
selaginella and isoetes the gametophyte does not escape from the spore,
nor does it form absorbing organs, nor develop assimilative tissue.
The reduced prothallium develops at the expense of food stored by the
sporophyte while the spore is developing. Thus, while the gametophyte
is separate from the sporophyte in selaginella and isoetes, it is
really dependent on it for support or nourishment.
=598.= The important general characters possessed by the ferns
and their so-called allies, as we have found, are as follows: The
spore-bearing part, which is the fern plant, leads an independent
existence from the prothallium, and forms root, stem, and leaves. The
spores are borne in sporangia on the leaves. The prothallium also leads
an independent existence, though in isoetes and selaginella it has
become almost entirely dependent on the sporophyte. The prothallium
bears also well-developed antheridia and archegonia. The root, stem,
and leaves of the sporophyte possess vascular tissue. All the ferns and
their allies agree in the possession of these characters. The mosses
and liverworts have well-developed antheridia and archegonia, and the
higher plants have vascular tissue. But no plant of either of these
groups possesses the combined characters which we find in the ferns and
their relatives. The latter are, therefore, the fern-like plants, or
_pteridophyta_. The living forms of the pteridophyta are classified as
follows into families or orders. (See page 295.)
=599.= TABLE SHOWING RELATION OF GAMETOPHYTE AND SPOROPHYTE IN THE
PTERIDOPHYTES.
--------------------------------------------------------------------
GAMETOPHYTE. (Becoming smaller, mostly independent.
In selaginella and isoetes becoming dependent on the sporophyte.)
---------------+-----------------------+----------------------------
| Vegetative Part. | Sexual Organs.
---------------+-----------------------+
Ferns. |A green, thin, | Usually both kinds on
(Polypodiaceæ.)|expanded, | the same prothallium.
|heart-shaped growth, +---------------+------------
|with rhizoids. |Antheridia with|Archegonia,
| |spermatozoids. |each with
| | |egg.
----------------+-----------------------+---------------+------------
Equisetum. |A green, thin, | Usually the two kinds
|expanded, | on different prothallia.
|lobed growth, +---------------+------------
||with rhizoids. |Antheridia, on |Archegonia
| |small male |on larger
| |prothallia, |female
| |with |prothallia,
| |spermatozoids. |each with
| | |an egg.
---------------+-----------------------+---------------+------------
Isoetes. |Colorless, rounded |
|mass of cells, |
|inside of spore wall, |
|usually no rhizoids, | On different prothallia.
|or but few. Two kinds. |
+-----------+-----------+---------------+------------
|Small ones,|Large ones,|One |Few
|male. |female. |antheridium, |archegonia,
|Developed |Developed |much larger |in apex
|into small |from |than the |of oval,
|prothallial|nutriment |single |colorless,
|cell, and |stored in |prothallial |female
|antherid |macrospore |cell. |prothallium,
|cell while |from |Antheridium |each with
|still in |sporophyte.|with |egg.
|sporangium.| |spermatozoids. |
---------------+-----------+-----------+---------------+------------
Selaginella. |Colorless, rounded |
|mass of cells inside |
|of spore wall, no |
|rhizoids, or but few. | On different prothallia.
|Two kinds. |
+-----------+-----------+---------------+------------
|Small ones,|Large ones,|One |Few
|male. |female. |antheridium, |archegonia,
|Developed |Developed |much larger |in apex
|into small |while |than the |of oval,
|prothallial|still in |single |colorless,
|cell, and |sporangium |prothallial |female
|antherid |and |cell. |prothallium,
|cell while |dependent |Antheridium |each with
|in |on |with |egg.
|sporangium.|sporophyte.|spermatozoids. |
---------------+-----------+-----------+---------------+------------
------------------------------------------------------------+-------
SPOROPHYTE. |Begin-
(Largest part of the plant. The fern plant. Independent | ning
of, and more hardy than, the gametophyte. | of
Usually perennial.) |Gameto-
| phyte.
------------+-----------+-----------------+-----------------+-------
| Beginning | | |
| of |Vegetative Part. | Fruiting Part. |
|Sporophyte.| | |
------------+-----------+-----------------+-----------------+-------
Ferns. |Fertilized |Root, stem, leaf.|Sporangia on |
(Polypod- |egg. | |leaf. All of one |
iaceæ.)|(Develops | |kind. Sporangium |
|into | | contains ....|Spores.
|fern | | |
|plant.) | | |
------------+-----------+-----------------+-----------------+-------
Equisetum. |Fertilized |Root, stem, leaf.|Sporangia on |
|egg. | |sporophylls. All |
|(Develops | |of one kind. |
|into | |Sporangium |
|equisetum | | contains ....|Spores.
|plant.) | | |
------------+-----------+-----------------+-----------------+-------
Isoetes. |Fertilized |Root, stem, leaf.|Sporangia of |
|egg. |Stem very short. |two kinds. Small |
|(Develops |Leaves bear |ones contain ....|Micro-
|into |sporangia in | |spores.
|isoetes |cavities at base;|Large ones |
|plant.) |outer leaves | contain ....|Macro-
| |usually bear | |spores.
| |macrosporangia, | |
| |inner ones | |
| |microsporangia. | |
------------+-----------+-----------------+-----------------+-------
Selagniella.|Fertilized |Root, stem, leaf.|Sporangia of |
|egg. |Spore-bearing |two kinds. Small|
|(Develops |leaves grouped |ones contain ....|Micro-
|into |on the end of | |spores.
|selaginella|stem in a spike. |Large ones |
|plant.) |Lower ones bear | contain ....|Macro-
| |macrosporangia, | |spores.
| |upper ones bear | |
| |microsporangia. | |
------------+-----------+-----------------+-----------------+-------
Classification of the Pteridophytes.
Of the living pteridophytes four classes may be recognized.
CLASS FILICINEÆ.[34]
This class includes the ferns. Four orders may be recognized.
=600. Order Ophioglossales.= (One Family, Ophioglossaceæ).—This
order includes the grapeferns (Botrychium), so called because of the
large botryoid cluster of sporangia, resembling roughly a cluster
of grapes; and the adder-tongue (Ophioglossum), the sporangia being
embedded in a long tongue-like outgrowth from the green leaf.
Botrychium and Ophioglossum are widely distributed. The roots are
fleshy, nearly destitute of root hairs, and contain an endophytic
fungus, so that the roots are mycorhiza. The gametophyte is
subterranean, and devoid of chlorophyll. In Botrychium virginianum,
an endophytic fungus has been found in the prothallium. Another genus
(Helminthostachys) with one species is limited to the East Indies.
=601. Order Marattiales= (One Family, Marattiaceæ).—These are
tropical ferns, with only four or five living genera (Marattia, Danæa,
etc.). They resemble the typical ferns, but the sporangia are usually
united, several forming a compound sporangium, or _synangium_.
The Ophioglossales and Marattiales are known as eusporangiate ferns,
while the following order includes the leptosporangiate ferns.
=602. Order Filicales.=—This order includes the typical ferns.
Eight families are recognized.
_Family Osmundaceæ._—Three genera are known in this family. Osmunda
has a number of species, three of which are found in the Eastern United
States; the cinnamon-fern (O. cinnamomea), the royal fern (O. regalis),
and Clayton’s fern (O. claytoniana). No species of this family are
found on the Pacific coast.
_Family Gleicheniaceæ._—These ferns are found chiefly in the tropics,
and in the mountain regions of the temperate zones of South America.
There are two genera, Gleichenia containing all but one of the known
species.
_Family Matoniaceæ._—One genus, Matonia, in the Malayan region.
_Family Schizæceæ._—These are chiefly tropical, but two species are
found in eastern North America, Schizæa pusilla and Lygodium palmatum,
the latter a climbing fern.
_Family Hymenophyllaceæ._—These are known as the filmy ferns because
of their thin, delicate leaves. They grow only in damp or wet regions,
mostly in the tropics, but a few species occur in the southern United
States.
_Family Cyatheaceæ._—These are known as the tree ferns, because of the
large size which many of them attain. They occur chiefly in tropical
mountainous regions, many of them palm-like and imposing because of the
large trunks and leaves. Dicksonia, Cyathea, Cibotium, Alsophila, are
some of the most conspicuous genera.
_Family Parkeriaceæ._—There is a single species in this family
(Ceratopteris thalictroides), abundant in the tropics and extending
into Florida. It is aquatic.
_Family Polypodiaceæ._—This family includes the larger number of
living ferns and many genera and species are found in North America.
Examples, Polypodium, Pteridium (= Pteris), Adiantum, etc.
=603. Order Hydropterales (or Salviniales).=—The members of this
order are peculiar, aquatic ferns, some floating on the water (Azolla,
Salvinia), while others are anchored to the soil by roots (Marsilia,
Pilularia). They are known as water ferns. The sporangia are of two
kinds, one containing large spores (macrospores) and the other small
spores (microspores). They are therefore heterosporous ferns.
_Family Salviniaceæ._—There are two genera, Salvinia and Azolla.
_Family Marsiliaceæ._—Two genera, Marsilia and Pilularia. In this
family the sporangia are enclosed in a sporocarp, which forms a
pod-like structure.
CLASS EQUISETINEÆ.[35]
=604. Order Equisetales.=—The single order contains a single
family, Equisetaceæ, among the living forms, and but a single genus,
Equisetum. There are about twenty-four species, with fourteen in the
United States (see Chapter XXIX).
CLASS LYCOPODIINEÆ.[36]
=605. Order Lycopodiales.=—The first two families of this order
include the homosporous Lycopodiineæ, while the Selaginellaceæ are
heterosporous.
_Family Lycopodiaceæ._—There are two genera. Lycopodium (club moss)
includes many species, most of them tropical, but a number in temperate
and subarctic regions. The gametophyte of many species is tuberous,
lacks chlorophyll, and in some there lives an endophytic fungus.
Phylloglossum with one species is found in Australia.
_Family Psilotaceæ._—There are two genera. Psilotum chiefly in the
tropics has one species (P. triquetrum) in the region of Florida.
_Family Selaginellaceæ._—These include the little club mosses, with
one genus, Selaginella (see Chapter XXX).
CLASS ISOETINEÆ.
=606. Order Isoetales=, with one family Isoetaceæ and one genus
Isoetes (see Chapter XXXI). There are about fifty species, with about
sixteen in the United States.
FOOTNOTES:
[34] As class Filicales in Engler and Prantl.
[35] As class Equisetales in Engler and Prantl.
[36] As class Lycopodiales in Engler and Prantl.