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Jan., 1905.] Heterosporous Pteridophyte. 255

THE LIFE CYCLE OF A HETEROSPOROUS PTERIDOPHYTE.J O H N H S C H A F F N ER .

The Heterosporous Pteridophytes represent the highest stageof development in the second or intermediate series of plants.The term heterosporous is applied to plants in which there aretwo k inds of nonsexual spores, large and small, called respectivelymegaspores and microspores. This peculiar spore condition isalso presen t in the seed plants . The megaspore always gives riseto a female individual and the microspore to a male. In thelower Pteridophytes there is only one kind of nonsexual sporesand they are, therefore, called homosporous. The HomosporousPteridophytes should be kept distinct from the Heterosporous,since the development of heterospory represents one of theadvancing waves of evolution which brought about profoundchanges in the charac ter of the vegetation of the ear th. Theelimination of chance env ironm ent in the development of uni-sexual individuals by predetermining the sex in the spore and thereduction in the size of the gam etophy tes appear to have beennecessary conditions in the evolution of the seed plants.The living species of Heterosporous Pteridophytes fall into sixgenera, the remnants of former extensive and dominant groups.The total number of species is about 635, somewhat more thanthe living Gymnosperms which constitute the next higher sub-kingdom . Because of their position as the lowest of heterospor-ous plants a thorough knowledge of their morphology and lifehistory is necessary to a proper understanding of the structuresand processes found in the Gymnosperms and Angiosperms.Nevertheless, much of their life history is still obscure and vari-ous statements rest on conjecture or imperfectly established facts.The living Heterosporous Pteridophytes are small herbaceousperennials or rarely annual plants, usually not more than a fewinches high. They are quite insignificant when compared withtheir ancient relatives some of which developed into great trees.The six living genera are named as follows: Azolla, Salvinia,Marsilea, Pilu laria, Isoetes, and Selaginella. Selaginella is, how-ever, a very complex genus. Azolla and Salvinia are small float-ing plants; Marsilea and Pilularia are creeping geophilous formsusually in wet places; Isoetes, known popularly as quillwort,grows in wet or swampy ground and has a short, upright, usuallysimple rhizome with grass-like leaves; Selaginella has very smallleaves on branching herbaceous stems which grow either hori-zontal or erect in wet ground or sometimes in dry places subjectto periodical moisture. These plan ts are usually placed in theclasses and orders of the Homosporous Pteridophytes, but sincethey represent a very important advance in the plant kingdom.

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256 The Ohio Naturalist. [Vol. V, No. 3,

a stage or step taken in several independent lines and by all thehigher groups, it seems best to put them together in a sub-kingdomof their own.The relationship of some of the included fossil forms is stillimperfectly understood on account of the absence of properlypreserved sporangia. The following orders are usually recog-nized :1, Calamariales; 2, Sphenophyllales; 3, Salviniales; 4, Mar-sileales; 5, Isoetales; 6. Lepidophytales; 7, Selaginellales. Thefirst, second, and sixth orders named above are entirely fossil.These seven orders fall naturally into five classes which may bedesignated as follows: Calamariae, Sphenophyllese, Hydropteridesor water ferns including the Salviniales and Marsileales, Isoeteaeor quillworts, and Selaginelleag, including the Selaginellales andthe fossil Lepidodendrids Sigillarids and other genera belongingto the sixth order.The Hydropterides are the only plants among the Heterospor-ous Pteridophytes that are leptosporangiate. In this respectthey are similar to some of the homosporous ferns. The resem-blance, however, does not extend much farther and it is not prob-able that they are a branch from the homosporous leptosporangi-ate ferns. The fossil record indicates that the water ferns aremuch the older group. It might be stated that the term lep-tosporangiate refers to the origin of the sporangium which inthese plants originates from an epidermal cell instead of fromhypodermal tissue as in all other higher plants. The Selaginellasare probably descended from the primitive Lycopods but thequillworts show no evident relationship to any known homospor-ous forms.Heterosporous Pteridophytes appear in the first known landflora, but these forms were not primitive types; for the primitivefloras have either not been discovered or else have left no fossiltrace of their existence. There is some evidence that members ofthis sub-kingdom were present in the Ordovician period; buthowever that may be they are definitely found in the Silurian andbecame very important in the Devonian. They culminated inthe Carboniferous, which from a botanical point of view might becalled the age of Heterosporous Pteridophytes. The coalswamps were full of great tree forms belonging to the genera,Lepidodendron, Sigillaria, Calamites, and others. These plantsformed the larger part of the material preserved as coal and weretherefore of great economic importance for the future welfare ofman. They declined during the Permian and very few appear tohave survived the great disturbance known in American geologyas the Appalachian revolution. These plants were essentiallymoisture-loving and when the great changes occurred which markthe transition from the Paleozoic to the Mesozoie they seem to

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Jan., 1905.] Heterosporous Pteridophyte. 257have been unable to adapt themselves to the new condition ofthings. The next higher subkingdom, the Gymnosperms, be-came the dominant group of plants.In tracing out the life cycle, Selaginella kraussiana will betaken as a representative with incidental references to othertypes. There is, of course, a well-marked alternation of genera-tions, the gametophyte and the sporophyte, and each generationusually leads an independent existence for at least a part of itslife. The conspicuous plant is the sporophyte or nonsexual gen-era tion. It consists of a more or less dorsi ventral leafy shoot

Fig . 1. D iagr am of Life Cycle of Selag inella.from the lower side of which dichotomously branched roots growou t. The roots strike ground usually after growing an inch ormore in length. In this plan t there is no increase in thickness ofthe stem but some Heterosporous Pteridophytes do grow consid-erably in thickness by the development of the general tissue butno t by a true cambium layer. The vascular bundles are con-centric with the xylem in the centre. The stem usually has twobundles side by side each contained in a tubular cavity or airspace and connected with the walls of the cavity by means ofnum erous slender filaments. Other types have a different stemstructure.After attaining a considerable size the sporophyte begins toreproduce by developing cones or primitive flowers at the ends ofsome branches. The cones are bisporangiate, having two sets ofsporophylls which are but slightly differentiated from the ordi-nary foliage leaves. The two kinds of spore-bearing leaves are

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258 The Ohio Naturalist. [Vol. V, No. 3,

called megasphorophyils and microsporophylls. There are num-erous microsporophylls in each cone but usually only one meg-asporophyll which is the lowest one of the set. The microsporo-phyll has a single microsporangium on the upper side in the axiland the megasporophyll also produces but one megasporangiumin the same position. In some of the other classes the two kindsof sporangia may be on the same leaf or leaf segment, which mayIbe closed up like the ovulary of a carpel. The microsporangiumand megasporangium produce the nonsexual spores, the micro-spores and the megaspores. The spores are produced in the fol-lowing manner: In the microsporangium there are a number ofcells called microsporocytes which become free in the cavity bythe dissolution of their walls. Each microsporocyte dividestwice giving rise to four cells. These cells develop into themicrospores. During the first division of the microsporocyte the-chromosomes of its nucleus are reduced so that the nuclei of thespores have only one-half as many chromosomes as were presentin the nuclei of the sporophyte or nonsexual plant. The mega-sporangium also contains a number of megasporocytes whichdivide in the same way as the microsporocytes and form a sporetetrad. During the first of the two divisions the chromosomesare also reduced from the 2% number to x. The chromosomesare small bodies which appear in the nucleus during division andare probably special bearers of hereditary tendencies. Thesereduction divisions of the sporocytes are of great importance inthe life cycle of the plant because of the profound changes whichtake place in the chromosomes. Only four megaspores developand they become so large that the megasporangium bulges outand becomes somewhat four-lobed. In Marsilea only one mega-spore develops in each megasporangium. The difference in sizebetween microspore and megaspore in Selaginella kraussiana isvery great. The ratio in volume in the mature condition is oftenmore than 1 : 2000. The small spore having only a limitedamount of cytoplasm and very little room for food material,always produces a male gametophyte when it germinates; whilethe megaspore always produces a female gametophyte. Thedetermination of the sex is apparently not primarily an inheritedcharacter but depends on the environment during the early stagesof embryonic development. Thus as stated above, the Hetero-sporous Pteridophytes by producing a difference of environmentin the spore are able to keep the two sexes distinct. Since thereare no hermaphrodite individuals there is no possibility of self-fertilization. This is true for all plants which have developedTieterospory. The ripe sporangia open by vertical slits and thespores are thus discharged. In Azolla the microspores are imbed-ded in a foamy mass of substance called a massula on whichpeculiar anchor-like appendages are developed. In Marsilea the

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Jan., 1905.] Heterosporous Pteridophyte. 259sporocarp or modified leaflet containing the sporangia, after beingsoaked for some time in water, opens in a very peculiar way bythe protrusion of a gelatinous ring containing the sori. Thisprocess represents an extreme specialization and adaptation to asemi-aquatic life.In Selaginella kraussiana the spores germinate sometimebefore they are discharged. The microspore has divided into twocells and the female gametophyte has developed to a considerableextent. This is a very interesting condition, since it representsone stage toward the development of seed plants in which thespores are not discharged and the gametophytes are completelyparasitic in the sporangia. In some Selaginellas the cones withthe sporangia and spores are shed.The gametophytes are very small especially the male. Thevegetative part of the male thallus is represented practically bya single cell. The remaining cells developed inside of the sporewall represent the antheridium or spermary. In some of theHeterosporous Pteridophytes the antheridium breaks through themicrospore wall, in others the spermatozoids escape through thebreak in the wall, no part of the male thallus protruding. Thecells of the antheridium are differentiated into peripheral or wallcells and two masses of central cells from which the spermatozoidsare developed. The spermatozoids are very small and have twoflagella. In the quillworts and water-ferns the spermatozoidsare multiciliate. Externally the male gametophytes of mostHeterosporous Pteridophytes look very much like the pollen-grains of the seed plants with which they are homologous.

The female gametophyte projects somewhat beyond the meg-aspore wall. It begins its development by free cell formationand later a layer of cells is formed in one side of the spore. Thisbreaks through the spore wall and a number of archegonia orovaries are produced, but the greater part of the spore cavity isfilled with irregular vegetative cells. The female thallus is des-titute of chlorophyll and is dependent upon the food laid up inthe spore. In Marsilea the female gametophyte is much morereduced having only a single archegonium whose neck projectsfrom one end of the megaspore. In its older stages the femalegametophyte may develop some chlorophyll. Each archegoniumas usual contains a single oosphere or egg which has been formedby the division of a mother cell. The sister cell of the egg, calledthe ventral canal cell, always dissolves. This small femalegametophyte is homologous with the so-calledrembryosac of theseed plants.While the gametophytes are lying on damp ground, and whencovered with water,^fertilization is accomplished. In someSelaginellas, however, fertilization occurs while the megasporescontaining the female gametophytes are still in the megasporan-

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z6o The Ohio Naturalist. [Vol. V, No. 3,gium. The spermatozoids swim through the water and enter thenecks of the archegonia, So far the Selaginellas are still depend-ent on an aqu atic condition. W hen a spermatozoid reaches theegg its nucleus unites with the egg nucleus and as a result thereis a double amount of chromation in the fertilized egg. Whenthe oospore germinates its nucleus produces twice as manychromosomes as were present in the cells of the pa ren t gameto-ph ytes . This conjugation or fertilization stage, therefore, rep-resents the second profound change in the life cycle of the plantand is just opposite in its results to the reduction division.The oospore is the first cell of the sporophyte generation. Itis not discharged bu t begins to divide by a transverse wall. Theouter cell develops into a suspensor while the inner gives rise tothe embryo proper. In other classes the embryogeny is quitedifferent and there seems to be much difference in the embry-ogeny of different Selaginellas. In the water ferns the develop-ment of the embryo is much the same as in the homosporousferns.

The embryo is pushed down into the centre of the mass offood cells in the lower part of the female gametophyte by therapid growth of the suspensor. It develops a foot, root, andstem tip with two small leaves called cotyledons. The foot occu-pies the cavity of the megaspore and takes up the food storedthe re. The root grows out, passes down into the ground, andbegins to take up water with dissolved mineral salts. The stemwith the cotyledons grows upward, develops chlorophyll, andthu s begins the manufacture of food. The embryo changes grad-ually from a phagophyte , nourished entirely by the female parent,to a holophyte, manufacturing its own food from the simple com-pounds taken from the earth and air. I t also passes graduallyfrom the enclosed condition to the external world, there being nosuch sudden change as the embryo undergoes during the sprout-ing of a seed.The little embryo sporophyte, having established relation-ships with the moist soil, air, and sunlight, continues to developinto a mature plant while the female gametophyte, its mother,dies. The gametophytes are short lived and are so reduced inbody that their life consists mainly in accomplishing the impor-tant process of fertilization and in assisting the sporophyte to geta proper start during its early and helpless, juvenile stage.