Liverwort: Alternation Of Generations (1974)

[Music] liverworts and mosses comprise a group of primitive land plants the bryophytes most bryophytes are found in humid places often besides streams and rivers to complete their life cycles they require abundant moisture though bryophytes are green and require sunlight direct light dries them hence they may grow on shaded tree trunks or the dark forest floor as we've said bryophytes can be classified into two groups one group is the true mosses bryophytes with stems and leaves unlike the leafy true mosses are the liverwurts the bodies of many liverworts do not have stems or leaves of the many kinds of liverworts one of the most familiar is marcantia marcantia may grow on damp soil or rock the plant body a phallus is small flat and green on its underside are hair-like projections rhizoids rhizoids perform root-like functions they anchor and absorb but unlike true roots each rhizoid is a long single cell of microscopic size rhizoids grow from the lower surface of the phthalus the upper surface of the phallus is covered with white dots a closer look reveals that these are pores which air may enter pore supplies a many sided chamber in which carbon dioxide is absorbed during photosynthesis liverworts reproduce bisexual then asexual means the sexual stage a multicellular gametophyte has gametes or sex cells the asexual stage a multicellular sporophyte has spores alternation between sexual and asexual reproduction between gametophyte and sporophyte is alternation of generations let's look first at the sexual generation a particular phallus being either male or female grows sex structures male sex structures are rounded female ones have scalloped edges time lapse photography shows how a female structure develops at maturity there are finger-like projections mature male structures are flat and disk-like male structures with their stalks are male branches a liverwort with these may grow some distance from one with female branches adaptations make sexual reproduction between separate plants possible to examine some adaptations we'll detach a female branch from the phallus we'll take a much closer look at its underside colorless membranes called envelopers protect archegonia female sex organs moving closer we see three flask-shaped archegonia with long necks and rounded bases each base contains a microscopic egg cell protected by structures of the female branch the male branch is somewhat different a cross section reveals no envelopers but many small pits on the top surface contain antheridia male sex organs antheridia are the large dark masses a single antheridium contains thousands of fertile male sex cells inside them are formed very active sperms one sperm developing in each fertile cell the following micro photography illustrates the increasing activity of maturing sperms each sperm has a large rounded head and two flagella at maturity a sperm opens a small pore in the cell wall through this it bursts from the cell now we more clearly see the flagella whip like appendages that make it possible for the sperms to move about at the end of each flagellum is a ball-like structure smaller than the head flagella may propel a sperm until it encounters an egg cell a sperm is 10 microns long swimming toward a female plant a foot away it travels the equivalent of a man walking about 40 miles to move liverwort sperms require moisture for many plant species the rainy season marks a period of intense growth raindrops falling on taller plants drip to the shaded ground where marketia lives in this season many liverworts complete the sexual or gametophyte parts of their reproductive cycles rainwater may collect atop a male branch soaking into the pits that contain antheridia it is in the presence of water that sperms become active the many sperms hundreds of thousands of them give the water a cloudy appearance the cloudiness spreads as sperms continue to be released a single drop of water on the top of a single male branch may contain many hundreds of thousands of sperms newly released sperms move without specific orientation in different directions in time droplets full of sperms either fall or are splashed from the male branches then entire groups of sperms move in the same direction with a sort of swimming action this action plus the movement of water along the ground may carry sperms for several inches or several feet until they encounter female plants surface tension of the water may form a bridge to the envelopers and archegonia an archegonium is entered by sperms of thousands of sperms only a few reach mature archegonia this is in spite of the fact that ripe archegonia secrete chemicals that attract sperms this attraction is chemotaxis to observe chemotaxis in the laboratory we dissect an archegonium from a female branch from a male branch we remove water containing sperms we place it on the archegonium and examine it through a high-powered microscope following sperms we see a general movement toward the archegonium experiments with similar sperms have shown that they are attracted by chemicals secreted by an archegonium this then is chemotaxis in time sperms enter the tip of the archegonium they travel through the long neck until eventually they reach the egg cell of the many sperms that may enter only one penetrates the egg cell fertilizing it a fertilized egg cell seen here in time lapse photography may move about for several hours finally about 50 hours after fertilization the egg begins to develop into an embryo the embryonic stage ends the sexual or gametophyte generation in the liverwort life cycle and begins the alternate asexual or sporophyte generation on the female branch under each finger-like projection they develop sporophytes small when compared to the gametophyte body of marcatia inside these are spore producing cells at first these cells are in chain-like order then meiosis the process of reduction division occurs when meiosis is complete each spore producing cell has given rise to four spores as spores mature the female branch opens in time it exposes the sporophytes in each sporophyte capsule further development takes place walls of the spores thicken and harden and twisted looking spring-like bodies form among them these are eleters structures capable of releasing spores from the capsule maturing sporophytes undergo changes seen here in time lapse photography a capsule emerges from the envelopers as its stalk grows at the same time the capsule's color changes from green to a deepening shade of yellow finally two months after the egg was fertilized the tip of the capsule opens irregularly and expansion and contraction of the eleters disperses the spores wind may distribute liverwort spores over several square miles each spore with its thick protective wall is a complete asexual reproductive unit which can grow into a new liverwort landing in a moist adequately light environment spores germinate immediately among the first signs of germination are the swelling of spores and the appearance of a number of chloroplasts a slender filament a protonema grows from each germinating spore as proto-nimata grow the plants approach the state of development we saw when we began the study of alternation of generations in the liverwort this is how a new gametophyte begins at the end of each filament a globular cell is formed the cell divides and as it does so the resulting mass of cells begins to look more like a phallus after some time rhizoids grow on the young gametophyte the phallus develops gradually assuming the flat shape of the mature gametophyte this cycle gametophyte sporophyte and gametophyte again illustrates alternation of generations in liverworts interestingly marcatia reproduces in other ways a phallus reproduces vegetatively in some instances by budding the buds or gemmy grow instructors known as jimmy cops jimmy cups grow on female plants as well as on males a very close look at a cup reveals the growing gemmy in it each looks somewhat like a small phallus as with liverwort sperm cells splashing rainwater activates jimmy and distributes them across the land water floats or splashes the gemmy from their cups gemmy from a female plant will grow into females those from a male into males jimmy may be widely dispersed by water deposited in a suitable place each of the gemmy develops into a new phallus though this phallus and others like it may reproduce vegetatively we know they also reproduce sexually and asexually as alternation of generations occurs reviewing alternation of generations in marketia we have found that the plant reproduces sexually by means of male and female sex cells or gametes and asexually by spores alternation of generations between multicellular gametophyte and sporophyte comprises the life histories of many groups of plants including liverworts and mosses the [Music] bryophytes bye