FURTHER STUDIES ON GYMNOSPERMS.
Cycas.
[Illustration: Fig. 366. Macrosporophyll of Cycas revoluta.]
=627.= In such gymnosperms as cycas, illustrated in the
frontispiece, there is a close resemblance to the members of the fern
group, especially the ferns themselves. This is at once suggested by
the form of the leaves. The stem is short and thick. The leaves have a
stout midrib and numerous narrow pinnæ. In the center of this rosette
of leaves are numerous smaller leaves, closely overlapping like bud
scales. If we remove one of these at the time the fruit is forming we
see that in general it conforms to the plan of the large leaves. There
are a midrib and a number of narrow pinnæ near the free end, the entire
leaf being covered with woolly hairs. But at the lower end, in place of
the pinnæ, we see oval bodies. These are the macrosporangia (ovules)
of cycas, and correspond to the macrosporangia of selaginella, and the
leaf is the macrosporophyll.
=628. Female prothallium of cycas.=—In figs. 367, 368, are
shown mature ovules, or macrosporangia, of cycas. In 368, which is a
roentgen-ray photograph of 367, the oval prothallium can be seen. So in
cycas, as in selaginella, the female prothallium is developed entirely
inside of the macrosporangium, and derives the nutriment for its growth
from the cycas plant, which is the sporophyte. Archegonia are developed
in this internal mass of cells. This aids us in determining that it is
the prothallium. In cycas it is also called endosperm, just as in the
pines.
[Illustration: Fig. 367. Macrosporangium of Cycas revoluta.]
[Illustration: Fig. 368. Roentgen photograph of same, showing female
prothallium.]
[Illustration: Fig. 369. A sporophyll (stamen) of cycas; sporangia in
groups on the under side. _b_, group of sporangia; _c_, open sporangia.
(From Warming.)]
=629.= If we cut open one of the mature ovules, we can see the
endosperm (prothallium) as a whitish mass of tissue. Immediately
surrounding it at maturity is a thin, papery tissue, the remains of the
nucellus (macrosporangium), and outside of this are the coats of the
ovule, an outer fleshy one and an inner stony one.
=630. Microspores, or pollen, of cycas.=—The cycas plant
illustrated in the frontispiece is a female plant. Male plants also
exist which have small leaves in the center that bear only
microsporangia. These leaves, while they resemble the ordinary leaves,
are smaller and correspond to the stamens. Upon the under side, as
shown in fig. 369, the microsporangia are borne in groups of three
or four, and these contain the microspores, or pollen grains. The
arrangement of these microsporangia on the under side of the cycas
leaves bears a strong resemblance to the arrangement of the sporangia
on the under side of the leaves of some ferns.
=631. The gingko tree= is another very interesting plant belonging
to this same group. It is a relic of a genus which flourished in the
remote past, and it is interesting also because of the resemblance of
the leaves to some of the ferns like adiantum, which suggests that
this form of the leaf in gingko has been inherited from some fern-like
ancestor.
[Illustration: Fig. 370. Zamia integrifolia, showing thick stem,
fern-like leaves, and cone of male flowers.]
[Illustration: Fig. 371. Two spermatozoids in end of pollen tube of
cycas. (After drawing by Hirase and Ikeno.)]
=632.= While the resemblance of the leaves of some of the
gymnosperms to those of the ferns suggests fern-like ancestors for the
members of this group, there is stronger evidence of such ancestry
in the fact that a prothallium can well be determined in the ovules.
The endosperm with its well-formed archegonia is to be considered a
prothallium.
=633. Spermatozoids in some gymnosperms.=—But within the past two
years it has been discovered in gingko, cycas, and zamia, all belonging
to this group, that the sperm cells are well-formed spermatozoids. In
zamia each one is shaped somewhat like the half of a biconvex lens, and
around the convex surface are several coils of cilia. After the pollen
tube has grown down through the nucellus, and has reached a depression
at the end of the prothallium (endosperm) where the archegonia are
formed, the spermatozoids are set free from the pollen tube, swim
around in a liquid in this depression, and later fuse with the egg. In
gingko and cycas these spermatozoids were first discovered by Ikeno and
Hirase in Japan, and later in zamia by Webber in this country. In figs.
371-374 the details of the male prothallia and of fertilization are
shown.
[Illustration: Fig. 372. Fertilization in cycas, small spermatozoid
fusing with the larger female nucleus of the egg. The egg protoplasm
fills the archegonium. (From drawings by Hirase and Ikeno.)]
[Illustration: Fig. 373. Spermatozoid of gingko. Some abnormal forms
have a tail. (After Ikeno and Hirase.)]
=634. The sporophyte in the gymnosperms.=—In the pollen grains
of the gymnosperms we easily recognize the characters belonging to the
spores in the ferns and their allies, as well as in the liverworts and
mosses. They belong to the same series of organs, are borne on the
same phase or generation of the plant, and are practically formed in
the same general way, the variations between the different groups not
being greater than those within a single group. These spores we have
recognized as being the product of the sporophyte. We are able then to
identify the sporophyte as that phase or generation of the plant formed
from the fertilized egg and bearing ultimately the spores. We see from
this that the sporophyte in the gymnosperms is the prominent part of
the plant, just as we found it to be in the ferns. The pine tree, then,
as well as the gingko, cycas, yew, hemlock-spruce, black spruce, the
giant redwood of California, etc., are sporophytes.
While the sporangia (anther sacs) of the male flowers open and permit
the spores (pollen) to be scattered, the sporangia of the female
flowers of the gymnosperms rarely open. The macrospore is developed
within sporangium (nucellus) to form the female prothallium (endosperm).
=635. The gametophyte has become dependent on the sporophyte.=—In
this respect the gymnosperms differ widely from the pteridophytes,
though we see suggestions of this condition of things in Isoetes and
Selaginella, where the female prothallium is developed within the
macrospore, and even in Selaginella begins, and nearly completes, its
development while still in the sporangium.
In comparing the female prothallium of the gymnosperms with that
of the fern group we see a remarkable change has taken place. The
female prothallium of the gymnosperms is very much reduced in size.
Especially, it no longer leads an independent existence from the
sporophyte, as is the case with nearly all the fern group. It remains
enclosed within the macrosporangium (in cycas if not fertilized it
sometimes grows outside of the macrosporangium and becomes green), and
derives its nourishment through it from the sporophyte, to which the
latter remains organically connected. This condition of the female
prothallium of the gymnosperms necessitated a special adaptation of the
male prothallium in order that the sperm cells may reach and fertilize
the egg-cell.
[Illustration: Fig. 374.
Gingko biloba. _A_, mature pollen grain; _B_, germinating pollen grain,
the branched tube entering among the cells of the nucellus; _Ex_,
exine (outer wall of spore); _P_₁, prothallial cell; _P_₂, antheridial
cell (divides later to form stalk cell and generative cell); _P_₃,
vegetative cell; _Va_, vacuoles; _Nc_, nucellus. (After drawings by
Hirase and Ikeno.)]
[Illustration: Fig. 375.
Gingko biloba, diagrammatic representation of the relation of pollen
tube to the archegonium in the end of the nucellus. _pt_, pollen tube;
_o_, archegonium. (After drawing by Hirase and Ikeno.)]
=636. Gymnosperms are naked seed plants.=—The pine, as we have
seen, has naked seeds. That is, the seeds are not enclosed within the
carpel, but are exposed on the outer surface. All the plants of the
great group to which the pine belongs have naked seeds. For this reason
the name “_gymnosperms_” has been given to this great group.
[Illustration: Fig. 376. Spermatozoids of zamia in pollen tube; _pg_,
pollen grain; _a_, _a_, spermatozoids. (After Webber.)]
[Illustration: Fig. 377. Spermatozoid of zamia showing spiral row of
cilia. (After Webber.)]
=637. Classification of gymnosperms.=—The gingko tree has until
recently been placed with the pines, yew, etc., in the order _Pinales_,
but the discovery of the spermatozoids in the pollen tube suggests
that it is not closely allied with the Pinales, and that it represents
an order coordinate with them. Engler arranges the living gymnosperms
somewhat as follows:
Class Gymnospermæ.
Order 1. Cycadales; family Cycadaceæ. Cycas, Zamia, etc.
Order 2. Gingkoales; family Gingkoaceæ. Gingko.
Order 3. Pinales (or Coniferæ);
family 1. Taxaceæ.
Taxus, the common yew in the eastern
United States, and Torreya, in the
western United States, are examples.
family 2. Pinaceæ. Sequoia (redwood of California),
firs, spruces, pines, cedars, cypress, etc.
Order 4. Gnetales. Welwitschia mirabilis, deserts of southwest
Africa; Ephedra, deserts of the Mediterranean
and of West Asia. Gnetum, climbers (Lianas),
from tropical Asia and America.
=638.= TABLE SHOWING HOMOLOGIES OF SPOROPHYTE AND GAMETOPHYTE IN
THE PINE.
TERMS CORRESPONDING TO THOSE USED IN PTERIDOPHYTES. COMMON TERMS.
-------------+--------------------------------------------------
| Sporophyte = Pine tree.
| Spore-bearing part = Male and female cones.
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Sporophyte | Microsporophyll = Stamen.
| Microsporangium = Pollen sac.
-------------+--------------------------------------------------
| Microspore = Pollen grain.
| Mature microspore is = Mature pollen grain.
| rudimentary male
| prothallium with
| rudimentary
| antheridium
| Large cell (part of = Vegetative cell
| antheridium wall?) of pollen grain.
| Antheridium cell = Small cell of pollen
Male | grain.
gametophyte | Antheridium cell divides = Generative cell.
| to form stalk cell and
| central cell of
| antheridium
| (male sexual organ)
| Central cell of = Paternal cells,
| antheridium or generative cells.
| divides to form
| two sperm cells
-------------+--------------------------------------------------
| Macrosporophyll = Ovuliferous scale (cover
| scale and carpellary
| outgrowth); or three
| carpels united into
Sporophyte | ovuliferous scale,
| the central one sterile
| (in axil of cover scale).
| Macrosporangium covered = Nucellus covered by
| by integument integument = ovule.
-------------+--------------------------------------------------
| Macrospore (remains in = Large cell in center of
| sporangium) nucellus which develops
| embryo sac and endosperm
| (remains in nucellus).
Female | Female prothallium = Endosperm, in nucellus.
gametophyte | (in sporangium)
| Archegonia (female = Corpuscula, in endosperm.
| sexual organs)
| Egg = Maternal cell, or germ
| cell.
-------------+--------------------------------------------------
| Egg (fertilized) = Germ cell.
| Young sporophyte = Pine embryo in nucellus
| and integument.
Young | Young sporophyte = Embryo |
sporophyte | In remains of = Endosperm |
| gametophyte = Nucellus | Seed.
| And sporangium = Integument |
| Surrounded by new |
| growth of old |
| sporophyte |
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