- The Bryophyta have a sharply defined alternation of generations in which the diploid asexual generation, although lllorphologically distinct from the haploid sexual generation, is always attached to it and wholly or partially dependent uporl it for nutrition. This condition is in contrast with that in Pterijiophyta and more advanced plants, where the asexual generation, the sporophyte, is always an independent plant at maturity.
- The sporophytes of plants higher than Bryophyta are internally differentiated into xylem and phloem; whereas Bryophyta lack these tissues.
- The sexual generation of Bryophyta, the gametophyte, is always an independent plant at maturity and one that is nutritionally self-sustaining because of the presence of chloroplasts within its body.
- In many cases the plant body is a thallus, that is, without differentiation into root, stem, and leaf. In those cases where it is externally differentiated into stem and leaf, there are never any roots; only one-celled absorptive organs (rhizoids).
- All Bryophyta are oogamous, and the gametes are produced within multicellular sex organs in which there is an outer sterile layer of jacket cells. In this respect they are immediately distinguishable from algae since sex organs of the latter, when multicellular, always have all cells gamete-producing.
- Life Cycle of Bryophyta: The life cycle of a bryophyte consists of a regular alternation of an asexually reproducing generation with a sexually reproducing one. This was first clearly demonstrated by Hofmeister in 1851,1 but the periodic doubling and halving in the number of chromosomes associated with the alternation was not clearly recognized until 1894.
- This alternation of generations is obligatory in the sense that a zygote resulting from gametic ~nion always grows into a sporophyte and that spores produced by a sporophyte always germinate to form a gametophyte.
- It is not obligatory in the sense that gametophyte and sporophyte must always alternate with each other, because vegetative propagation of the gametophyte may result in a life cycle in which there is a succession of gametophytic individuals before there is the formation of a sporophyte.
- It should also be noted that vegetative budding does not always result in a generation similar to the parent one. Thus, vegetative budding of· gametophytic tissue may ;result in the formation of a sporophyte. This production of a sporophyte from a gametophyte and without any union of gametes is apogamy. Conversely, a sporophyte may bud off a mass of cells which develops into a gametophyte. Production of a gametophyte from a sporophyte without the formation of spores is apospory. Apogamy and apospory are of rare occurrence in Bryophyta as compared with Pteridophyta.
- Most of the recorded cases among bryophytes are among the Musci where a wounding of sporophytes induces an aposporous production of gametophytes. The Anthocerotae are also known to be aposporous.
- Origin of the Bryophyta. The Bryophyta are undoubtedly a series of great antiquity, but the paleontological evidence in support of this contention is very meager. This is not surprising when one takes into consideration the delicate tissues of the bryophytes and the consequently poor chance of their becoming fossilized. However, anacrogynous Jungermanniales are known to have been present in the middle Carboniferous.
- The presence of Musci in the upper Carboniferous is not established with as great certainty, 5 but there seem to be good reasons for believing that moss-like plants existed at that time. Fragments of typical Sphagnum leaves have been found in the Cretaceous.
- Although it is generally agreed that the Bryophyta have arisen from the green algae, the manner in which they have arisen is a matter of speculation since there is a wide gap between the Chlorophyceae and the simplest known bryophyte. It is generally agreed that the progenitors of the Bryophyta belonged to the ulotrichaceous series of the Chlorophyceae.
- Typical Ulotrichales are filamentous and have a plant body primarily fitted for an aquatic existence.
- The Bryophyta are largely’ terrestrial, and those of them that are aquatic in habit, as Riella, are land plants in which the aquatic habit is secondarily acquired. The transfer of algae from an aquatic to a terrestrial environment is thought to have lead to a development of a more massive plant body in which the reduced surface, in proportion to the volume, tended to prevent undue drying of the thallus. The primitive Bryophyta, similar to many present-day species, probably grew where there was an abundance of moisture in the soil. Thus there was but little evolution of water-absorbing tissues, except for the elongation of certain of the cells on the under surface into rhizoids. The abundance of moisture also resulted in a retention of the method of gametic union typical of algae, that is, by one or both of a pair of uniting gametes moving freely in all directions by swimming through water. The persistence of this primitive method of gametic union among all pteridophytes and in certain of the lower gymnosperms is one of the striking features in the evolution of plants.
- Origin of the Sex Organs. Although it is relatively easy to visualize the manner in which the plant body of an alga may be evolved into a plant body such as is found in bryophytes, it is much more difficult to envisage how the sex organs typical of Bryophyta have evolved from those of algae.
- The advantage of the sterile jacket layer present in sex organs of Bryophyta is obvious-it protects the delicate gametes from undue drying as they develop. The need of this protection is shown by the widespread occurrence among Bryophyta of additional features
protecting the sex organs against loss of water. These include: embedding of the organs within the thallus; the development of accessory envelopes; and the surrounding of the organs by leaves.
- The most attractive hypothesis explaining the origin of the sex organs characteristic of bryophytes holds that archegonia and antheridia are fundamentally alike and that they arose by sterilization of the outermost cells of a multicellular gametangium, similar in general appearance to the gametangium of Ectocarpus.
- One of the weaknesses of the hypothesis is that such gametangia, although widespread among brown algae, arenot known for any green algae. However, certain green algae, as Schizomeris, produce reproductive cells in a structure essentially like a multicellular gametangium, and others, as Chaetonema, have multicellular antheridia resembling the gametangia of Ectocarpus.
- The gametangial hypothesisl holds that the hypothetical ancestral green algae were probably heterogamous and had both sorts of gametes motile In both the male and the female gametangia the outermost cells lost their gamete-producing capacity and matured into a sterile layer that enclosed the fertile cells.
- Retention of the gamete-producing capacity of all the fertile cells within the male gametangium resulted in the antheridium typical of Bryophyta.
- Archegonial evolution was accompanied by a sterilization of all but the lowermost of the interior cells. The single fertile cell lost the power of motility and became an egg. According to this interpretation the neck canal cells and ventral canal cell of an archegonium are homologous with the first formed fertile cells within an antheridium.
- The evidence supporting this hypothetical origin of bryophytan sex organs comes largely from the abnormal sex organs occassionally developed by various Hepaticae and Musci.
- These reversionarysex organs are almost always archegonia. In some of them interior cells other than the lowermost become fertile.
- Sometimes these cells mature into eggs, but more often they divide and redivide to form masses of cubical cells identical in appearance wich those within antheridia.
- Reversion of the superficial cells to a fertile condition is most unusual, only one or two such cases having been recorded.
- Origin of the Sporophyte. Two theories have been advanced to account for the dissimilar alternating generations of Bryophyta and plants higher in the evolutionary scale.
- According to the homologous theory,4 the sporophyte is a direct modification of the gametophyte and not an entirely new structure.
- Advocates of this theory hold that the sporophyte is to be interpreted as a neutral generation, modified from the sexual generation, and as one especially adapted for spore formation. The direct proliferation of one generation from the other in apogamous and aposporous individuals is the chief argument in favor of this theory.
- The antithetic theory holds that the sporophyte is an entirely new structure intercalated in the life cycle. According to this theory the zygote, instead of immediately undergoing a reduction division and forming spores, divided to form a number of diploid cells, each of which divided reductionally and formed spores.
- The result was a rudimentary type of sporophyte in which all the cells were sporogenous. From this was evolved a sporophyte in which the superficial cells matured into sterile cells instead of into reproductive cells.
- The Ricciaceae are the only present-day Bryophyta with this simple type of sporophyte.
- Further evolution of the sporophyte came about as a result of additional sterilization of the sporogenous tissue. In the higher Bryophyta this has resulted in a sporophyte differentiated into foot, seta, and capsule.
- The theory that there has been this progressive sterilization of sporogenous tissue rests largely upon the comparison of supposedly primitive and advanced sporophytes.
- Evidence supporting this assumption is also to be seen in abnormal individuals where, as in Porella, the internal tissues of seta and foot revert to a fertile condition and form spores.
- Green algae showing beginning of an evolution toward the simple Riccia type of sporophyte are unknown.
- At one time Coleochaete, with its formation of several zoospores by the zygote, was thought to show this, but it is noW known that the first division of the zygote nucleus of Coleochaete is reductional and that there is nothing corresponding to. a rudimentary
- The origin of a diploid asexually reproducing generation seems to be best explained by the antithetic theory, that is, by an intercalation of vegetative cell divisions between gametic union and meiosis. However, such an origin of the sporophyte does not explain the dissimilarity of the two generations among Bryophyta. The dissimilarity of the two
is probably associated with the retention of the zygote within the archegonium and the resultant parasitism of the intercalated generation upon the sexual generation. On the other hand, parasitism may not be the sole cause of the dissimilarity since many brown algae have free-living alternating haploid and diploid generations that are dissimilar in
- Three major evolutionary series (Hepaticae, Anthocerotae, Musci) are recognizable among the Bryophyta, but their relationships one to another are obscure.
- All three series appear to have diverged early from the hypothetical primitive Bryophyta evolved from the Chlorophyceae
- From the standpoint of structure of the sporophyte, the Hepaticae are nearest to the hypothetical ancestors since they include the genera with the simplest known sporophytes.
- Evolution among the Hepaticae has been chiefly in the gametophytic generation and has been either an elaboration of external complexity with a retention of internal simplicity, or has been. an elaboration of an internal complexity with a retention of external simplicity.
- Most sporophytes of Hepaticae show considerable sterilization of sporogenous tissue, but none of them shows any tendency toward unlimited (indeterminate) growth. For this reason alone, the Hepaticae may be cO,nsidered an evolutionary sideline that ends blindly.
- The Musci also seem to be a blindly ending evolutionary sideline.
- Their gametophytes are externally complex and with some internal differentiation of tissues. ‘Most of them have sporophytes with a greater internal differentiation of tissues than do those of any Hepaticae, but growth of all sporophytes of Musci is also definitely limited. The Musci are sometimes thought to have been evolved from the leafy Hepaticae (Jungermanniales, Calobryales), but this appears improbable when ontogeny of sex organs of the two and the early embryology are takep. into consideration.
Gametophytes of the Anthocerotae, except for their embedded sex organs, are less advanced than those of Hepaticae and Musci. On the other hand, the sporophytes are of a much more advanced type in that their growth is indeterminate.
- Their indeterminate growth is thought to be a feature of fundamental importance which has made possible an evolution of a pteridophytic type of plant.
- Classification of Bryophyta. The conventional treatment of the Bryophyta is to range them in two classes, Hepaticae and Musci. Within the past quarter century there has been a growing tendency to accept the proposal1 that the Anthocerotales be placed in a class coordinate with the Musci and Hepaticae. When the Anthocerotales are placed in a separate class, the Bryophyta are divided into the three following classes:
- The Hepaticae which have gametophytes that are dorsiventrally differentiated
and either externally simple or externally differentiated into leaf and stem. The interior of the gametophyte may be homogeneous or composed of various tissues. Sex organs, except when terminal in position are always formed from superficial cells on the dorsal side of the , thallus. The sporophyte may be simple or with a foot, seta, and capsule. In either case, it is determinate in growth and entirely parasitic upon the gametophyte.
- The Anthocerotae which have gametophytes that are dorsiventrally differentiated, of simple external form, and internally homogeneous. The male sex organs are formed from hypodermal cells on the dorsal side of the thallus. The sporophyte is differentiated into a capsule and foot. The lower portion of a capsule is meristematic and continually adding to the upper portion. Sporogenous tissue in a sporophyte arises from the outer layer (amphithecium) of an embryo.
- The MUsCi which have gametophytes with a transitory prostrate stage bearing sexual branches which continue growth as independent plants after disappearance of the prostrate portion. The sexual branches are differentiated into stem and leaves, radially symmetrical, and the sex organs are formed from superficial cells. The sporophyte consists of a foot, seta, and capsule. Frequently it has an internal differentiation of the sterile tissues and is only semiparasitic upon the gametophyte.
Growth of the sporophyte is always determinate.
- The Hepaticae which have gametophytes that are dorsiventrally differentiated
CRYPTOGAMIC BOTANY (VOLUME II)
BY: GILBERT M. SMITH (Stanford University)