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of Viscunty of some species of Loranthus^ and of Rhizophora :
as the result of developmental study of the latter it is stated
by Warming (Engler, Jahrb. Bd. 4. p. 519) that the multi-
locular condition is easily explained by arrest of development
of parts of the sporogenous tissue, and formation of sterile
septa. Probably a similar developmental explanation will be
found to apply in certain other cases, e.g. Aegiceras^ Phajus^
Bletia^ and Rafflesia^ all of which have multilocular anthers.
Since such septation resulting from partial sterilization is thus
shown to occur in pollen-sacs of Angiosperms, it will, I think, be
futile to deny the possibility of its having occurred also in lower
forms : the question therefore becomes one o{ probability only.
It will probably be remarked that this occurrence of



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352 Bower. — A Theory of the

septation among Angiosperms is but sporadic: that it is
merely an 'adaptive' character, and that the sporadic
occurrence of it deprives it of systematic importance. In
answer to such objections I would say that to me all per-
manent morphological characters are probably at one time or
another * adaptive,' though we may distinguish between those
which are results of relatively recent adaptation and those
which were relatively primitive. It may be that the same
method of advance may have brought in its train important
physiological advantages at one point of evolutionary history :
but when repeated at a later period the advantages may have
been less weighty : the result would be in the one case relative
permanence, in the other less permanence. This I conceive
to be the case for septation of sporangia in the homosporous
Arch^oniatae as compared with septation of anthers in
Angiospermic plants.

But while we thus recognize that parts of the sporogenous
tissue may form sterile septa, I have been able to show that
the converse may also take place ; viz. that tissue of a septum,
which is normally sterile, may on occasions form spores.
This occurs in certain abnormal synangia of Tnusipteris, and
is, in my view, a reversion. In Fig. i6i of my memoir in the
Philosophical Transactions ^, a synangium of almost normal
form is seen : but though the position of the usual septum is
clearly indicated, the cells there are already assuming the
character of tapetum or of sporogenous cells, as is more
clearly seen in Fig. i6a, where they are repre^nted in a draw-
ing on a larger scale. This conversion of the sterile septum
into sporogenous tissue is more conclusively shown in Fig. 164,
in which the development is slightly more advanced, while
Fig. 165 represents a part of the contents about the line ( x )
where the septum should normally be: here it is seen that
sporogenous cells form a continuous chain across the line, thus
conclusively proving that the normally sterile septum has
become sporogenous.

The conclusion to be drawn from such observations, together

* See Pfoc. Roy. Soc, No. 326, 1893.



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Strobilus in Archegoniate Plants. 353

with the facts of formation of septaabove noted, will be, that there
is no fundamental difference between sterile tissue of a septum and
sporogenous tissue^ since both appear to be mutually convertible.
Lastly, I would point out that the arrest or sterilization
may involve a whole sporangium. If the strobili of species
of Lycopodium or SelagineUa be examined, it will be found
that, passing downwards from the base of the strobilus,
sporangia are to be seen in the normal position, but succes-
sively of smaller size, till a small sterile group of cells is all
that represents the sporangium : finally the sporangium dis-
appears altogether. A similar state of things is found at the
apex. Again, in such species as L. Selago there are suc-
cessive sterile and fertile zones, which graduate off into one
another through zones showing such successive series of
abortive sporangia as those above noted. Similar imperfect
bodies (synangia) are frequently found at the limits of the
fertile zones in the Psilotaceae, while the fertile spike of
Ophioglossum may also be represented by a small non-fertile
body. How are these facts to be regarded from an evolutionary
point of view? Are the small non-fertile bodies nascent
germs of sporangia, or are they vestigial ? As a third alter-
native it may be suggested that they have no phylogenetic
bearing whatever ; but considering the frequency of their
occurrence, and the gradual steps of their arrest, I do not
think this last view to be a probable one. I think we
can only consider them to be vestigial organs: potential
sporangia, arrested, and in the down-grade of development.
Their occurrence is doubtless very closely connected with the
physiological position of the plant which bears them, but the
recognition of this does not in any way explain away the
interest which attaches to their frequent presence. The leaves
which subtend the sporangia (sporophylls) differ in many
species of Lycopodium from those which do not (vegetative
leaves), though their relation to the axis is the same : in
passing from the strobilus to the vegetative region a gradual
transition from sporophylls to the vegetative leaves is seen,
and it proceeds parallel with the arrest and final disappear-



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354 Bower. — A Theory of the

ance of the sporangia. If the arrested sporangia be accepted
as vestigial, then the correlatively larger foliage-leaves which
subtend them must be regarded as sterilized sporophylls, and
the conclusion follows that in some cases at least foliage-leaves
are sterilized sporophylls.

Before leaving this subject it may be remarked that a similar
view has been applied by Prantl and others to the stamens
and perianth-whorls of Ranunculaceae, &c., viz. that the
latter owe their origin to sterilization of staminal members.
It seems not unlikely that this view may be applicable also in
many other cases among the higher plants.

We have thus arrived at the position that sterilization of
potential sporogenous tissue is a common phenomenon, re-
curring frequently throughout the Arch^oniatae and Phanero-
gams. We find it aflFecting single cells, groups of cells, or
even whole sporangia. In certain cases the sterile cells may
remain isolated, or be absorbed by the developing spores,
thus appearing to serve a directly nutritive function : in other
cases permanent sterile rods (trabeculae, or columellae) or
plates of tissue may be formed, and there is reason to think
that a grouping together of sterile cells may have resulted in
certain cases in the formation of sterile tissue-masses (Antho-
ceroteae). In others, again, complete septa may partition off
a previously concrete potential sporogenous mass into isolated
portions : such tissue-masses as trabeculae and septa may
serve the purposes of mechanical support, and of assistance in
bringing nutrition to the masses of developing spores. We
have further learned from the resumption of spore-production
by certain sterile septa, coupled with the facts of sterilization,
that there is no fundamental difference between sterile septa
and fertile tissue, either being convertible into the other.
With these conclusions before us, drawn from both higher
and lower forms, we may now address ourselves to the
problem of forming some idea of the probable methods of
morphological advance of the simpler homosporous Vascular
Cryptogams. On grounds of comparison it has been gene-
rally held that they originated from some Algal-Bryophytic



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Strobilus in Archegoniate Plants. 355

ancestry, and I see no sufficient reason for doubting this view.
I am fully impressed by the great remoteness of their origin



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356 Bower. — A Theory of the

stages are over. The advantages of direct nutrition, and of
perennation of the vegetative system are, from the point of
view of spore-production, obvious enough, and a greatly
increased output of spores became thereby possible.

The second point, viz. the substitution of discrete arche-
sporia for one concrete one would also be a great biological
advantage : as the size of the single sporogenous mass in-
creases, the difficulty of supply of food to all the developing
spores makes itself increasingly felt, while further the risk of
mechanical damage becomes more serious: for during the
semi-fluid condition of the contents of the sporangium, where
all the sporogenous cells are separate from one another, a large
sporangium is mechanically ill-protected, while a single punc-
ture by animal or other agency would ruin the whole. Again,
separate loculi may be matured in succession, thus not making
a simultaneous demand for nutrition, while keeping up a supply
of spores for an extended period. From the point of view of
nutrition and protection, therefore, septation would appear to
be a biological advantage.

Thirdly, from the point of view of dispersal, a projection of
the sporangia beyond the general surface is clearly a gain:
the mechanical difficulties of scattering the spores from pro-
jecting sporangia being much less than from deeply seated
ones. Thus on various grounds we should be prepared to
expect septation and separation of sporangia to appear in
increasing degree in an ascending evolutionary series.

We shall next inquire if among living Vascular Cryptograms
there is any evidence which would support the idea tliat
septation has taken place : and the answer is to be found in
the frequent occurrence among them of synangia, such as
those of Tmesipteris^ Psilotuniy Danaea^ Marattia, Kaulfussia^
Ophioglossum. Moreover, the sporangia, when distinct from
one another, are so grouped in many Pteridophyta as to
suggest a common origin from a synangial body by separation
and rounding off of the individual sporangia : this is the case
with the son of many Leptosporangiate Ferns. The develop-
ment of the sporangiophore of Equisetum suggests a similar



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Sirobilus iti Archegoniate Plants. 357

view as the explanation of its origin also.* Hitherto the con-
verse view has commonly been entertained, viz. that synangia
are the result of coalescence of sporangia in the course of
descent from an ancestry with separate sporangia ; in fact, the
whole morphology of homosporous Pteridophytes has been
dominated by the belief that the Leptosporangiate Ferns are
the nearest of vascular plants to the Bryophyta. I have
already stated at length elsewhere my reasons for thinking
that view to be ill-founded (Annals of Botany, Vol. V. p. 109) :
however firmly convinced any readers may be of the correct-
ness of the belief that the Leptosporangiates were the most
primitive Ferns, and the Ferns the most primitive of vascular
plants, I would ask them for the moment to relinquish that
opinion, and contemplate with me an alternative view.

First, I would state the opinion that, in the course of
evolution, simple and small-leaved forms preceded complex
and large-leaved forms; accordingly, unless there be strong
reasons against it, we shall be prepared to seek among small-
leaved, strobiloid, homosporous Pteridophyta for those which
reflect most nearly the primitive condition.

Secondly, it would appear that, unless there be strong
evidence to the contrary, synangia should be recognized as
the result of septation. The examples above cited from the
anthers of Angiosperms were treated by writers on the subject
in the right way ; partly from comparison of allied forms, and
partly from the study of development, it was concluded that
the anthers had become septate owing to a partial sterilization
of potential sporogenous cells. The same has been concluded
in the case of the trabeculae of Isoetes. A similar course
should be taken with the synangia of Psilotaceae, Filicineae,
and Ophioglossaceae ; but I would premise that in plants
where, as in these^ the meristems are not disposed in definite
strata, the recognition of a potential archesporium as a definitely
limited and continuous band of tissue must not be too
rigorously demanded before the synangia be admitted as
results of septation. The facts which have been acquired
relating to the Psilotaceae have already been stated at length



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358 Bower, — A Theory of the

elsewhere (Phil. Trans. 1894), and those relating to the
Ophioglossaceae are nearly mature ; in both cases the develop-
mental facts support a viewof septation. It appears to me
that morphologists have in the past been too ready to talce
refuge in hypotheses of reduction, as applied to the homo-
sporous Arch^oniatae. It has been assumed, with, as
I think, too little reference to the biological position of the
organisms, that synangia were the result of coalescence of
originally distinct sporangia. I hold that, on considerations
of development, comparison, and biological position, there is
good reason to believe that certain at least of the plants
bearing synangia were on the up-grade of evolution, and that
septation has been a factor in producing them as we now see
them.

There is one further factor which it will be necessary to
discuss, as contributing to the advance of the sporophyte from
similar beginnings, viz. the origin of appendicular parts. The
origin of the root may be dismissed as not directly affecting
our present discussion, and there will remain the question of
origin of such parts as are included under the terms sporan-
giophore, sporophyll, and foliage-leaf. It is conceivable that
there may have been various modes of origin of such parts,
and that they are not all truly comparable as regards their
descent ; but I think there is good reason to believe that at
least one mode of origin was by a process of eruptiofi from
a hitherto smooth surface. This suggestion is in no way
subversive, but falls in with observed facts of the origin and
development of leaves in vascular plants generally ; whether
we take the strobilus or the vegetative shoot, the ontogenetic
origin of the appendicular parts might be described as eruptive.
What I suggest is, that the phylogenetic history of sporophylls
was similar to the history of the individual as we now see it.
It is easy to bring forward examples of analogous eruption
of new parts of a higher order from the smooth surfaces of
other parts ; for instance, in the development of simple foliage-
leaves the margins are occupied by smooth wings; in com-
pound leaves, however, these smooth surfaces show an eruptive



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Strobilus in Archegoriiate Plants. 359

upgrowth of papillae, which develop into the pinnae ; more-
over, the whole of the wing-surfaces is not occupied by the
eruptive growths, so that the smooth and the eruptive
conditions may often be seen at different regions of the wings
of the same foliage-leaf. Again, in certain flowers, as in the
Hypericaceae and Myrtaceae, the originally smooth surface
of the staminal growths shows an eruptive development of
numerous stamens. Thus, whether in the vegetative or floral
r^ions, eruptive formation of new parts from a smooth surface
is known. I have elsewhere contended for a consistent
morphological treatment of the shoot throughout (Phil.
Trans. 1884, Part II). Botanists recognize the appearance of
pinnae by an eruptive process on the margins of the leaf, and
would, I presume, admit that simple leaves preceded compound
ones. What I suggest now is that the origin of the sporan-
giophore or sporophyll in the descent of Vascular Cryptogams
from some simpler Archegoniate forms was by a similar
eruptive growth from the smooth surface of a body of the
nature of a sporogonial head.

Having thus cleared the ground, and having recognized as
possible factors in the advance a process of septation by
formation of sterile septa from a previously continuous arche-
sporium, and an eruption of the surface so as to produce
appendicular organs, I will now briefly state certain views
at which I have arrived as regards the morphology of the
Pteridophj^, but the detailed production of facts will have
to be given elsewhere.

Fixing attention mainly on the spore-producing region^
since for reasons already noted the sporogenous tissues are to
be regarded as primary, we see on tiie one hand among the
Bryophyta that the spores are produced on sporogonial heads,
with a central sterile columella, surrounded by a continuous
archesporial layer, and protected by an external wall. Select*
ing some small-leaved, strobiloid Vascular Cryptogam, we see
the strobilus performing the same function of spore-production»
but with different superficial conformation. Centrally is the
sterile tissue of the axis, the internal part of which may be

Bb



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360 Bower. — A Theory of the

compared with the columella ; the continuous archesporium of
the Bryophyte is replaced by the discrete archesporia of the
Pteridophyte, while these are carried out on appendicular
organs, the sporangiophores. Thus the strobilus would appear
to be the correlative of some body like a sporogonial head, in
which the archesporium is septate and borne outwards on
eruptive growths. Applying these ideas to the strobilus of
Equisetunty we see the probability of them reflected in the
development. The surface of it when young is almost smooth :
the isolated cells which are to form the archesporia, can be
recognized almost as soon as the surface becomes undulated
by the eruption of the sporangiophores: these cells are
proportionately nearer the surface than in the Bryophyta:
but it has already been pointed out that the very formation
of the columella was a step in the direction of relegating the
spores to a superficial position, while such a position is
necessary for the dispersal of the spores from small loculi.
I do not mean to suggest that by such comparisons as these
the hypothesis of origin of a strobilus from a body of the
nature of a sporogonial head can be proved : the comparison
may be, and indeed probably is, merely a tracing of analogies
in parts which have advanced along somewhat similar lines :
and our endeavour is, as explained at the outset, not so much
to trace homologies as to recognize the methods of advance
in archegoniate plants: the chief points which have been
recognized thus far, and are believed to have been the
important factors in advance are (i) sterilization of potential
sporogenous tissue^ (a) formation of septa, (3) relegation of the
spore-producing cells to a superficial position, and (4) eruption
of outgrowths {sporangiophores) on which the sporangia are
supported.

In the Lycopods also the whole strobilus may be recognized
as the result of similar methods of advance : regarding Phyllo-
glossum as probably their simplest representative, we see that
the plant consists of the protocorm, bearing protophylls,
which may have been the result of direct vegetative out-
growth from the protocorm. Borne upon an elongated



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Slrobilus in Arckegoniate Plants, 361

peduncle is the simple strobilus: its conformation does not
differ very materially from that of Equisetum^ beyond the
smaller number of the sporophylls, and the fact that each
bears but one sporangium upon its upper surface, instead of
a number affixed all round. From Phylloglossum the advance



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362 Bower, — A Theory of the

sto'ile tissue projecting upwards into the sporangium, though
more efficiently in the Psilotaceae by formation of complete
septa. But the septation which is there seen in a minor
d^ree becomes a more prominent feature in the Ophio-
glossaceae, a family which on general grounds of comparison
of both generations I believe to be allied to the Lycopods.
I cannot here enter into details of the evidence (see Phil.
Trans. 1894), suffice it to say that I still consider the facts
sufficient to support the conclusion advanced some years ago :
that septation has resulted in the production of the fertile
spike of Ophioglossum from a sporangium of Lycopodinous
type. The other genera Botrychmm^ and Helminihosiachys
may be regarded as showing further steps in the separation
of distinct sporangia, the latter exhibiting also an eruption
of sporangiophores, laterally on the fertile spike : a repetition
in fact of the same process as we have recognized in the origin
of the strobilus itself. This great elaboration of the body
which appears to correspond to the Lycopodinous sporangium,
marches parallel with the increase of the subtending sporophyll :
in this series, which I believe to be an advancing series, I think
we may thus see how characteristically large-leaved forms,
with relatively few leaves expanded in slow succession, may
have arisen from strobiloid forms with numerous small leaves.
This leads to the third large series of Pteridophyta, viz.
the Ferns : I think it not improbable that they, with their
large leaves and numerous sporangia, originated from some
smaller-leaved strobiloid ancestry. On grounds already
explained elsewhere (Annals of Botany, Vol. V, p. 109)
I have concluded that the Eusporangiate Ferns were probably
the more primitive, and of these perhaps Danaea the most
so. It is not difficult to understand how, on a hypothesis of
septation of sporangia spread out over the surface of an
enlarging sporophyll, the peculiar sorus of that Fern might
originate : from such a type, by divers methods of isolation
of the sori, and separation of the sporangia, the various forms
of Leptosporangiate Ferns may have been derived. So far
then from accepting the latter as the primitive types of



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Strobtlus in Archegoniate Plants. 363

vascular plants, I should rather regard them as later deri-
vative forms, characterized by an extravagant growth of the
leaf and its appendages.

The chief points which the Ferns have in common with
the Ophioglossaceae are the large, often compound leaf, and
numerous, often separate, sporangfia. It is easy to imagine
the origin of these two families by parallel development from
a smaller-leaved ancestry : in the one case the extension and
septation of a definite sporangium of Lycopodinous type
would result in the so-called fertile frond of the Ophio-
glossaceae, this enlarging separately from the subtending leaf:
in the other the spore-producing body would throughout be
closely connected with the enlarging leaf -surf ace and be spread
out over it, as we see to be the case in Danaea. The method
of advance in complexity would be virtually the same in both
cases, viz. by septation, and subsequent separation of the
several sporangia. But evidence on such points as these is
but of the slenderest.

If this theory be applicable to the strobilus of Vasculaf
Cryptogams, it should also be so to the flower of Phanero-
gams. At present I do not propose to pursue the matter
further in this direction, beyond saying that I see no valid
objection in the way, while the recognition of the Phanero-
gamic flower as a strobilus of ultimate origin like that of
Vascular Cryptogams would make certain difliculties of floral
morphology appear less serious. But though the Phanero-
gamic flower be accepted as the homologue of the strobilus, it
must never be forgotten that while the homosporous strobili
are entirely non-sexual, the flower of Angiosperms is in its
development intimately connected with the sexual function.
The presence of this important factor in the one, and its
absence in the other makes it difficult to draw physiological
comparisons between them as regards the conditions which
would produce or modify them.

In previous attempts to explain the origin of the complex
strobilus of vascular plants, some idea of terminal branching
similar to that branching which is occasionally seen in abnormal



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364 Bower.—- A Theory of the

Moss-sporogonia has been held. The result of this would be
that the apex of the sporophyll would compare with the apex
of a sporogonium, or of some branch of it In suggesting that
the whole strobilus is the equivalent of some body of the nature
of a sporogonial head, clearly the apex of the one will correspond
to the apex of the other, while the eruptive members will be
mostly or all lateral.

In entertaining this theory, we shall necessarily part company
with the old views of metamorphosis, and I must state, with all
distinctness, that the opinion that the strobilus is a result of
modification of a vegetative shoot is, to my mind, incompatible
with our present views as to descent of plants constantly
maintaining an antithetic alternation, in which spore-production



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