Origin of the Gimnosperms and Seed Habit

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The Origin of Gymnosperms and the Seed HabitAuthor(s): John M. CoulterSource: Botanical Gazette, Vol. 26, No. 3 (Sep., 1898), pp. 153-168Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/2465288

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I54 BOTANICAL GAZETTE [SEPTEMBERform of its gametophyte body, a third to the structure of itssporophyte body. Any attempt to relate these to one anotherupon the basis of a single structure, even so important a one asthe sporogonium, is essentially misleading. But when we con-sider the totality of structure, we are led to the opinion thatthese lines possibly diverged from an archetypal plexus in whichthere were gametophyte bodies as simple as that of Aneura, andsporophyte bodies as simple as that of Riccia. Another illus-tration is the recent attempt of Arnoldi to associate Isoetes withSelaginella largely upon the basis of endosperm development,without regard to great diversities in habit and anatomicaldetails. The association may be perfectly proper, but thereason given for it is inadequate.

In dealing with problems of phylogeny it is also importantto remember that the origin of a prominent group of livingforms from another group of living forms is extremely improb-able. We can point out resemblances in structures which wehave come to regard as essential, but this is not likely to meanthe origin of the one group from the other. It may mean thatthe two groups can be traced to one, probably now extinct,which combined the characters-now differentiated. Most livinggroups are best regarded as divergent rather than consecutiveseries.

But even this apparently sure ground has become veryuncertain from the fact, becoming more and more apparent, thatsimilar changes in structure, even very important ones, may haveappeared independently in different lines. The response oforganisms in structure to their environment is deeper seatedthan we were once inclined to believe, and testimony from thesimilarity of certain structures, when contradicted by the major-ity of other structures, argues feebly for recent community oforigin. Such similarities in structure argue more for physiolog-ical conditions than for phylogeny. For instance, from thestandpoint of evolution, the appearance of heterospory amongthe pteridophytes is one of the most important contributions toplant progress made by the group, but it is impossible to escape


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i8981 THE ORIGIN OF G YMNOSPERMS 155the conclusion that heterospory was attained independently byseveral lines. To put into the same genetic group all hetero-sporous pteridophytes would be regarded as a morphologicalabsurdity. If heterospory appeared independently in severallines, the same conclusion must be reached in reference to itsnatural outcome, the seed, and the polyphyletic origin of thespermatophytes becomes extremely probable.

This increases the perplexities of phylogeny, but it broadensits horizon, and introduces another possibility. To continue thesame illustration, in our search for the origin of seed-plants wehave narrowed attention to the existing heterosporous pterido-phytes, when some of the spermatophyte groups, as for examplethe gymnosperms, may represent an entirely distinct line inwhich heterospory and then the seed appeared, and may not berelated directly to any existing heterosporous pteridophyte. Insuch a case we are permitted to look to some group of livinghomosporous pteridophytes as possibly containing the best liv-ing representatives of the group from which gymnosperms havebeen derived.

With all these possibilities in mind, I wish to discuss thephylogeny of the gymnosperms, not so much to reach a clearphylogeny, as a clearer understanding of the complexity of theproblem, and the uncertainty of conclusions. This is a field inwhich no one can afford to be dogmatic.

THE ORIGIN OF GYMNOSPERMS.From Hofmeister's classic researches to the discovery of

gymnosperm spermatozoids by Hirase, Ikeno, and Webber, thefact has become increasingly apparent that gymnosperms arevery closely related to pteridophytes. It was natural, for atime, to regard gymnosperms as phylogenetically intermediatebetween pteridophytes and angiosperms, for it was not easy tobelieve that such a structure as the seed appeared in more thanone genetic line; but it is probably not going too far to saythat there is now no serious opposition to the view that thegymnosperm and angiosperm lines are genetically independent.


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I56 BOTANICAL GAZETTE [SEPTEMBERHowever, such a discussion does not lie within the scope of thispaper.

That gymnosperms have been derived from pteridophytestock is hardly open to discussion, at least we must assume thatthis is true, or all attempts at phylogeny are useless. The firstquestion which confronts us, therefore, is whether the verydivergent gymnosperm lines have had a common origin in thispteridophyte stock or not. Was there a single group of archaicgymnosperms, derived from pteridophytes, which subsequentlydifferentiated into distinct lines.? The existing gymnospermgroups are so very diverse that one of two things seems evident:either they differentiated into divergent lines from a commongymnosperm stock in very ancient times, or they originatedindependently from the pteridophyte stock. From this discus-sion I wish to exclude the Gnetales, as we do not possess suffi-cient data concerning their early history, or concerning themorphology of the very dissimilar living forms, to justify anyopinion as to their origin. They are such dissimilar fragments,living in such extreme conditions, that their origin is totallyobscure. In some respects they are more cycad-like than coni-fer-like, but in most respects they are so unlike both that a sep-arate origin seems possible. It may be even true that the threegenera belong to groups of independent origin, which is cer-tainly the easiest way of disposing of their differences; andtheir common characters of true vessels, the so-called perianth,and elongated micropyle, may have been attained independentlyas readily as was heterospory; but the combination of charactersin common does not seem to justify such a disposition of them,and the three genera had better be regarded as of commonderivation, wonderfully diversified by ancient separation, isola-tion, and extreme conditions.

Approaching the subject from the historical standpoint, thegreat group Cordaites seems to be the first with sufficient datato justify consideration. The structure of the vascular bundles,especially those of the leaves, is said to suggest those of coni-fers, cycads, Isoetes, and Ophioglossum; and the sporophylls


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1898] THE ORIGIN OF GYMNOSPERMS I57are organized into a strobilus, a character common to pterido-phytes and gymnosperms. But such characters can be usedonly as cumulative testimony. In such evidences as we have ofthe structure of the male gametophyte, however, we obtainsome valuable suggestions. Within the mature microsporethere appears a considerable group of polygonal cells. In liv-ing groups of gymnosperms, so far as investigated, there is nosuch structure; and if we look to pteridophytes for suggestion,we are constrained to believe that this group of cells is eitherprothallial or sperm mother cells. In either event, it wouldrepresent a condition of things much nearer pteridophytes thanis shown by any living seed plant. In view of the discovery ofspermatozoids in Cycas, Zamia, and Ginkgo, taken in connectionwith the peculiar structure of the male gametophyte justdescribed, I am of the opinion that the Cordaites also devel-oped spermatozoids. With either hypothesis as to the natureof the cells developed within the microspore of Cordaites, inseeking for the pteridophyte origin of the group, we are ledaway from such heterosporous pteridophytes as now exist, forin them the male gametophyte is much more reduced than inCordaites, in fact, more reduced than in most living cycads andconifers.

Additional testimony to the same effect is furnished by sec-tions of the seeds of Cordaites. In addition to the remarkablenucellus beak, which probably has no phylogenetic significance,the large pollen chamber is the most conspicuous feature. Thisis sometimes so extraordinarily large that it occupies the wholeupper portion of the nucellus, and has been observed to containnumerous pollen grains. The pollen chamber is a well-knownCycad feature, and seems to be associated with the early devel-opment of siphonogamy. By means of it, the tubular outgrowthfrom the antheridium wall is reduced to a minimum, and maycoexist with spermatozoid development, as shown by Hirase,Ikeno, and Webber.

The testimony all indicates that in Cordaites we have thebeginnings of a siphonogamic line, brought about by the reten-


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158 BOTANICAL GAZETTE [SEPTEMBERtion of the megaspore, which still develops its exine in Cor-daites and some cycads.

As to the pteridophyte group from which the Cordaites werederived, data are not sufficient to make opinion other than apure hypothesis. I think it is clear that such heterosporouspteridophytes as are living today must be set aside in thissearch, by the testimony of both of their gametophytes, espe-cially the male. They stand for lines which have very muchreduced the male gametophyte, have variously modified thefemale gametophyte, but have not developed siphonogamy byretaining the megaspore. It may be that the lycopod forms ofthe Carboniferous and earlier formations represent the pterido-phyte plexus from which Cordaites were derived, but we knowtoo little of their morphology to make any assertion. My judg-ment is that the Cordaites represent an independent hetero-sporous line, and that if they were associated in origin with thelycopod forms at all, it was before the latter had developedheterospory, which seems never to have been extensively devel-oped in the lycopod line until recent times.

I believe that we must regard either the ancient homosporouslycopod forms or the abundant Palkeozoic Marattia forms asresponsible for the origin of Cordaites, and my own inclinationis toward their Marattia origin, perhaps for no better reason thanthat in such an origin I see more opportunity for the develop-ment of such a group as cycads; but such a view is further sup-ported by the discovery that the spermatozoids of cycads, andtheir ally, the ginkgo, are of the multiciliate type, and not bicil-iate, as in living lycopod forms. Just what stress should belaid upon this I do not know, but when opinion is fairly balancedit would seem to help to a decision. It seems satisfactory,therefore, to regard the origin of cycads as from the homospor-ous-eusporangiate plexus of Filicales, represented today mostabundantly by Marattia and its allies. It would seem furtherthat this has been brought about without the intervention ofsuch Cordaites as we recognize, which, with probably similarorigin, were developing a very different type of body, that


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I898] THE ORIGIN OF G YINOSPERMS 159finds its modern expression in the conifers. In the acknowledgedCordaites, therefore, I recognize a transition region between thehomosporous-eusporangiate plexus of Filicales and the moremodern conifer series; while in the cycads we have a line whichcontinued more of the fern habit and structure, recognizable notmerely in its foliage leaves and general port, but in its occasionalvascular bundles of concentric type, and its multiciliate sperma-tozoids. The Cordaites, however, must have included forms thatwe have not recognized as such, for it is only when they becomedifferentiated from the fern habit that in the main we are ableto distinguish them. This very fact of their sharp differentiationmeans that they had made a decided departure, and we are prob-ably able to recognize only the most highly specialized forms. Ofcourse, in what I have said I may have been using the nameCordaites in a much more inclusive sense than taxonomy wouldjustify. As ordinarily defined I would see in them the first dis-tinct beginnings of a type which afterwards gave rise to theconifers; as used in this paper, they refer to a plexus of formsderived from the homosporous-eusporangiate Filicales which gaverise to both cycads and conifers as divergent lines, one retainingmore nearly the fern habit and structure and culminating earlier,the other departing more widely from the habit and structureand culminating later. I believe that some Pakeozoic formsnow regarded as ferns will be found to be more closely relatedto the Cordaites. How many other lines arose from this largeCordaites plexus, as I have defined it, we have no means ofknowing, but it seems to be responsible at least for all of theliving gymnosperm forms.

It is important to obtain such historical evidence as we canin reference to the gymnosperm lines, restricted in this paperto the Cordaites, conifers, and cycads. If a historical sequencecan be established which conforms to the views expressed hereas to the interrelationship of these lines, the conclusion willhave additional support. I need not apologize for the paucityof data furnished by paleobotanists. They have done what theycould, and we are greatly in their debt. Morphologists recog-


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i6o BOTANICAL GAZETTE [SEPTEMBERnize, however, that the structures usually preserved are not themost convincing as to relationships, and that nowhere are appear-ances more deceitful. While we have no sympathy with wildgeneralizations based upon fragmentary material, there is anincreasing accumulation of data which furnish a substantialfoundation for some conclusions. It seems to be clear that dur-ing the Pakeozoic there was an increasing display of gymno-sperms. The fragments which bear this testimony became veryabundant in the later periods of the Paleozoic, and are regarded,for the most part, as Cordaites. Associated with these forms isthe great display of Marattia and its allies. A distinct type ofleaf and of stem is attributed to each of these great groups,and when seeds or sporangia are associated with them the caseseems clear enough, but apart from such association the uncer-tainty is profound. Intergrading forms between the two are tobe expected, but with material so fragmentary and non-commit-tal it would be a rare chance that would lead to its definitedemonstration. In the Coal Measures the cycad type becomesapparent, but not prominent. This would seem to indicate eitheran early differentiation from the Cordaites plexus, or a late dif-ferentiation from the Marattia plexus. I see no difficulty in theformer view, as I see no advantage in multiplying the independ-ent heterosporous and seed lines until forced to do so by incon-trovertible evidence. The domination of cycads during theMesozoic, and their subsequent decline are well-known facts.

More suggestive, however, is the history of the conifers. Itis generally stated that this line, in its modern expression, beganduring the Paleozoic, and that our modern genera have beenrecognized by stem and leaf anatomy. Such methods of deter-mination we know to be untrustworthy, as there is the greatestpossible amount of anatomical diversity even in contiguousregions of the same organ, much more in different organs andat different ages. In examining the claim that modern conif-erous genera appeared during the Coal Measures, I find no evi-dence that seems to be worthy of serious consideration except-ing that with reference to Ginkgo, and it is an interesting


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I898] THE ORIGIN OF G YMNOSPERMS I6ifact that Ginkgo is no longer regarded as a conifer. Longbefore the evidence of spermatozoids was discovered it seemedperfectly clear to me that Ginkgo was more cycad-like thanconifer-like. In the light of our present knowledge the appear-ance of Ginkgo in association with the Carboniferous cycadsseems natural enough. It is a matter of very secondary impor-tance whether we are to regard it as an independent line or not.I am inclined to believe that while during the Palkeozoic hetero-spory and the seed were both attained, siphonogamy was inits beginnings, and that the spermatozoid habit was for themost part still continued in the seed lines. There is no conclu-sive evidence, therefore, that any of our modern coniferousgenera appeared during the Paleozoic. during which the Cor-daites were the dominating seed plants. During the last Palaxo-zoic periods undoubted conifers did appear, and in considerableabundance, and we may recognize the beginnings of distinctlines represented today by Abies and its allies, Taxodium andits allies, and Taxus and its allies, but the genera are not thoseof today. In the lower Mesozoic, however, modern araucarianand abietinous genera appear; and the Taxodium and Taxuslines become more distinct, but not modern until the later Meso-zoic. At that time Cupressus forms also appear, but not ofmodern genera. Further details are not necessary, as the point tobe made is that the conifer type was not recognizable until late inthe Paleozoic, and then not in its modern expression. It cer-tainly suggests a later departure from the Cordaites stock thando the cycads.

Another fact is interesting to note in connection with theevolution of the conifer forms. In existing conifers there isconsiderable variation in the development of the male gameto-phyte. In some forms, as the Abietineaw, the development oftwo or three prothallial cells, distinct from the large antheridialcell, is a well-known fact, an amount of prothallial develop-ment not shown by any other living heterosporous forms, eventhe heterosporous pteridophytes. In other forms, as Cupres-sineac and Taxeae, the reduction of the male gametophyte is


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I62 BOTANICAL GAZETTE [SEPTEMBERgreater, no sterile prothallial cells appearing, the whole struc-ture being an antheridium, as in the angiosperms. Our historicalevidence accords with this progressive reduction of the malegametophyte, the Taxus and Cupressus lines having attainedmodern expression after the Abies line; and back of the Abies linewe find the Cordaites, with probably a still greater development ofthe sterile region of the male gametophyte indicated. To derivethe Cordaites or Abies lines, with their two or three to many-celled sterile tissue of the male gametophyte, from such hetero-sporous lycopod forms as we know today, with their constantlymore reduced male gametophytes, is not within the bounds ofprobability. Besides, the reduction of the male gametophyteseems to be so prompt a response to heterospory, that its par-tially reduced condition in certain conifers, and probably in Cor-daites, would seem to argue for their near derivation from somehomosporous type.

The development of a suspensor in the lycopod forms hasalso suggested a genetic connection with gymnosperms, in whichthe suspensor development is so conspicuous. This organ, how-ever, seems to have no morphological constancy. In gymno-sperms it may be developed from a plate of cells formed in theoospore, as in most conifers; or from a mass of cells formedbasally or parietally in the oospore, as in cycads; or from freecells formed within the oospore, as in Ephedra; or from theelongation of the oospore itself, as in Gnetum; or from thedownward elongation of the archegonium, as in Welwitschia.The suspensor, therefore, seems to be a temporary organ ofthe embryo, of various morphological origin, intended to relatethe embryo properly to its food supply, and not of phylo-genetic significance.

The testimony of history and morphology seem to combinein pointing to a very generalized Paleozoic type as the origin ofgymnosperms. This type is characterized by its advancementtowards seed production rather than by its habit, which musthave been extremely varied to have given rise to such types ascycads and conifers. The usually recognized Cordaites show


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1898] THE ORIGIN OF GYMNOSPERMS i63but one tendency of a much more extensive group, for which thename Cordaites may be extended for convenience. Cordaites inthis larger sense occur in such association with groups of homo-sporous eusporangiate Filicales, and approach them so muchnearer in the important morphological structures mentioned thanthey do living heterosporous Filicales, that an independentheterosporous line is suggested. If such be the case, in thepassage from the Marattia forms to the Cordaites form bothheterospory and the retention of the megaspore were attained,and probably siphonogamy begun.

THE SEED HABIT.The evolution of heterospory seems simple enough. The

physiological differentiation of the spores was complete whenprothallia became persistently dicecious. This division of laboris to be expected in the case of two such distinct functions asthe production of antheridia and archegonia. A prothalliumproducing both sex organs equally well may be regarded as ina state of equilibrium, an equilibrium which is disturbed by anyconditions which favor the production of one sex organ ratherthan the other, in this case probably nutritive conditions. Thisdisturbance of the equilibrium of a bisexual prothallium wouldcertainly find an expression first in a dicecious tendency, andfinally in a dioecious habit. With the habit once fixed the mor-phological differentiation of spores becomes inevitable, since thenutritive requirements of the two prothallia are so different.The evolution of heterospory seems to be one of the simplestof selective processes, with inequalities of nutrition to furnishthe variations. From this point of view it would seem naturalto expect that it may have been derived frequently from homo-spory.

The retention of the megaspore, however, does not seem to beso simple a problem. In a certain sense it is correlated with thereduction of the gametophyte, since retention would not seempracticable until reduction had proceeded far enough to makethe gametophyte endosporic. Even greater reduction, however,


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I64 BOTANICAL GAZETTE [SEPTEMBERis attained by the male gametophyte, but the spore is shed. Itshould be noted that even in the case of the microspore the malegametophyte is usually completely organized before pollination;but the fact remains that the reduction does not compel reten-tion. It has seemed to me that this phenomenon is to beexplained by Bower's law of sterilization, developed in referenceto the strobilus. This law certainly finds expression in themegasporangia of heterosporous pteridophytes, in which thesterilization of mother cells is conspicuous. This method ofincreasing the nutrition of the fertile cells is too common aphenomenon to need illustration; but it is a tendency that wouldseem very consistent with the development of megaspores,whose peculiar work holds so definite a relation to abundantnutrition. For this very reason high numbers of microsporesmay be continued, and a diminishing number of megasporesproduced. This would reach its culmination in the productionof but a single megaspore by a sporangium, and a proportionateincrease in the size of the megaspore. With the developmentof a single spore imbedded in a sterile tissue, shedding becomesnot only mechanically difficult, but meaningless, since the neces-sity of scattering a brood of gametophytes, to avoid competi-tion, has disappeared. It is further true that the developmentof such a spore involves nutritive supplies from numerousneighboring cells, and a certain amount of retention becomesnecessary for this reason. Still further, the advantage to asingle megaspore in being retained, thus securing more abundantoutside nutrition during germination, would fix the habit if anyselective process were at work. For these various reasons it wouldseem evident that when the sterilization of a megasporangiumhad reached its extreme limit, by organizing a single spore,retention is likely to follow sooner or later. If this line ofreasoning be true, the seed habit might have been developed inany heterosporous line.With the retention of the megaspore pollination becamenecessary, but its gymnosperm expression differs in no way fromthe scattering of aerial spores in all the lower groups. The new


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i8981 THE ORIGIN OF GIMNOSPERMS i65feature demanded by the retention of the megaspore, therefore,was not the scattering of the microspores, but the developmentof siphonogamy. That the first retained megaspores wereexposed to the microspores can hardly be doubted, and in suchcases we now know that the spermatozoid habit must have beenretained, and that no tube, or a very small protuberance of theantheridium wall, was needed to discharge the spermatozoidssufficiently near the oosphere. If chemotropism can explainthe guidance of a pollen tube through much interveningtissue, it would certainly be sufficient to cause the protrusionof an elastic antheridial wall. In the very few illustrationsof Cordaites obtained, the megaspore is but slightly covered bysterile tissue at the bottom of a deep pollen chamber, and a veryslight development of tube is necessary. The same condition iscontinued in the cycads, and thus the habit of siphonogamymay have been gradually built up. As siphonogamy developed,the gradual failure of the sperm mother cells to organize sper-matozoids followed, and presently, almost exclusively now ingymnosperms, sperm mother cells are found to function directlyas male gametes, without further organization.

The secondary results which followed the retention of themegaspore were numerous. The well-known effect of fertiliza-tion upon adjacent tissues necessarily involved at least thesporangium, and the seed resulted. The presence of abundantavailable nutrition and favorable conditions induced the imme-diate germination of the oospore, which the development of aresistant tissue about the sporangium checked. As a conse-quence, the development of the embryo was thrown into twostages, the intra-seminal and the extra-seminal.

In the case of the angiosperms, however, another tendencywas connected with the retention of the megaspore, namely, thetendency of the sporophyll to enclose the megasporangium, atendency so evident in such pteridophytes as Isoetes and Mar-silea, that the direct pteridophyte origin of the group seemsmore natural than an origin from so specialized a type as thegymnosperms. Given the reduction of spore production to a


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i66 BOTANICAL GAZETTE LSEPTEMBER

single megaspore and the persistent enclosure of the sporan-gium by the sporophyll, and the angiosperm peculiarities follow.The profound effect of these conditions upon the germination ofthe megaspore is so remarkable, and intergrading stages so com-pletely unknown, that there seems to be no clue to the sequenceof changes. That an endosporic gametophyte might eliminatethe archegonium seems evident, for the tendency is shownamong gymnosperms by Gnetum, where oospheres are organizedby free endosperm cells. That the reproductive region of thefemale gametophyte may be organized earlier than the nutritiveregion, when the gametophyte is supplied with outside nourish-ment by the retention of the megaspore, is hinted at among theheterosporous pteridophytes and gymnosperms. These tend-encies have found full expression in the angiosperms, wherearchegonia have disappeared and the reproductive tissue of thefemale gametophyte is persistently organized before the nutritivetissue. Evidence as to the details of the evolution of this tend-ency is lacking, and may not be in existence, but the tendencyhas certainly reached a remarkably definite expression. Theunvaried appearance and movement of eight free nuclei or cells,and the remarkable fusion of two of them, represent habits sofixed through such an enormous group that they baffle explana-tion, and argue both for the monophyletic origin of angiosperms,and against their derivation from so divergent a line as gymno-sperms.

The earlier evolution of the gymnosperm line is probably tobe explained by ecological conditions. The body as a rule isorganized to endure extreme conditions. It is certainly not amesophytic type, and its evolution was certainly not in responseto prevailing mesophytic conditions. On the contrary, theangiosperm type is essentially a mesophytic one, with great foli-age display, and probably expanded in response to widelyprevalent mesophytic conditions. This might explain the habitpeculiarities of the two groups, but whether the more reconditemorphological differences hold any relation to these or not is tooobscure to permit even speculation.


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I898] THE ORIGIN OF GYMNOSPERAIMS I67SUMMARY.

i. A great Cordaites plexus, more extensive than the oneusually included under that name, represented the characteristicPakeozoic seed plants.

2. It was probably derived from homosporous-eusporangiateFilicales, represented today most abundantly by the Marattiaforms and their allies, and was the most common Paleozoictype of Filicales.

3. From it the gymnosperm lines, at least the cycads andconifers, were derived, the usually recognized Cordaites repre-senting a transition stage towards conifers.

4. The frequent independent appearance of heterospory is tobe expected, as it probably results from inequalities of nutri-tion in connection with the development of antheridia andarchegonia.

5. The retention of the megaspore, resulting in the seedhabit, follows the extreme sterilization of the megasporangium,which is attained with the organization of but one megaspore.With the development of a single megaspore imbedded insterile tissue, shedding becomes mechanically difficult, unnec-essary, and even disadvantageous from the standpoint of nutri-tion.

6. The retention of the megaspore was followed by thedevelopment possibly of seed coats, through the well-knowneffect of fertilization upon adjacent tissues; by immediate germi-nation of the oospore, on account of the favorable conditionsand the abundant supply of available nutrition; and by thechecking of the developing embryo by the mature seed struc-tures, resulting in the characteristic intra-seminal and extra-seminal stages of germination.

7. The first retained megaspores were doubtless directlyexposed to the microspores, and in Cordaites and cycads a pol-len chamber of varying depth and extent is associated with the


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i68 BOTANICAL GAZETTE [SEPTEMBERearly stages of siphonogamy, with which spermatozoid habit wasmore or less associated.

8. The pollination of gymnosperms is but a continuation ofthe ordinary method of dispersing aerial spores employed bycryptogams, the chief result of the retention of the megasporeupon the male gametophyte being the development of siphonog-amy.

THE UNIVERSITY OF CHICAGO.

Origin of the Gimnosperms and Seed Habit

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