Life Cycles

Land Plants

- Again, all land plants have an alternation of generations; that is, a life cycle alternating between gametophyte (haploid) and sporophyte (diploid) generations. - In addition, all land plants have multicellular sporangia and gametangia. - Sporangia are multicellular structures in whichspores are produced, by meiosis; sporangia occur only on sporophytes, which have diploid bodies. - Spores germinate and develop into gametophytes. - -- Gametes fuse to become a zygote (inside the archegonium), which in land plants develop into a sporophyte. - Gametangia are multicellular structures in which gametes are produced, by mitosis in land plants; gametangia occur only on gametophytes, which have haploid bodies. - Two types of gametangia are produced by land plants in general (lost in some seed plants): (1) archegonia are gametangia that produce an egg; (2) antheridia are gametangia that produce sperm. - Land plants produce an embryo (young sporophyte formed before rapid growth begins; embryos can be long-dormant in seed plants) - seen in all land plants, which are descendants of green algae. Not all green algae have an alternation of generations but some do.

Moss life cycle

- In the land plants called bryophytes (mosses, hornworts, and liverworts), the sporophyte remains attached to the gametophyte throughout its life and is dependent on the gametophyte parent to at least some extent (and the sporophyte is relatively short-lived). - In bryophytes such as mosses, the gametophyte generation is said to be dominant; dominance in the sense of life cycles means the generation that is largest, most complex, most long-lived, and/or most independent nutritionally. - Note that the moss life cycle shows several of the features noted earlier as adaptations to land, such as multicellular sporangia, resistant spores, gametes protected by gametangia during development, and protected embryos, as well as an innovation for aerial spore dispersal (the peristome), but still requires standing water for fertilization (for sperm to swim to archegonia), as in the aquatic, green algae.

Female-gametophyte formation; pollination; fertilization and embryo formation; resulting in mature seed (in pine)

- Male and female pine gametophytes are each sheltered from the environment, although male gametophyte must reach immature seed (= ovule), by wind - Free-standing water is not required to complete the life cycle - Ovule/seed protects and nourishes the entire female gametophyte generation and the subsequent sporophyte generation during its early development (as embryo and even seedling), even after dispersal

Fern Life Cycle

- Most ferns have a life cycle like the one shown here, with one type of gametophyte bearing both types of gametangia (antheridia, which produce sperm, and archegonia, which produce eggs). - The sexual life cycle in ferns requires free-standing water for sperm to be able to swim to the archegonia, as in the bryophytes; this need places some constraints on fern ecology, although asexual reproduction (and desiccation tolerance) allows some ferns to live in deserts. - The gametophyte generation, as in other modern vascular plants, is very small (but still visible to the naked eye here) and simple (although it is free-living and photosynthetic in most ferns) -- it is highly magnified in the drawing here; - in contrast, the sporophyte generation is conspicuous and relatively large, although it is dependent on the gametophyte initially (but only briefly, before becoming completely independent). - In ferns, sporangia are usually found on the underside of leaves in clusters (sori) and most have means of forcefully ejecting spores. In other words, the sporophyte is the dominant generation in ferns, unlike in bryophytes (such as mosses).

gymnosperm life cycle

- Sporophyte-dominant alternation of generations again (as in all vascular plants) - the gametophyte generation in seed plants includes two different kinds of gametophytes (male & female); that is, the gametophyte generation is unisexual (and each type of gametophyte developed from a different type of spore --- a microspore or megaspore, borne in microsporangia or megasporangia, which in turn are borne in separate cones in a pine -- pollen cones and seed cones, respectively). - Having unisexual gametophytes militates against self-fertilization. - Such separation of sexes in the gametophytes allowed for the evolution of the seed (and pollen). - Also, free-standing water is no longer necessary for sperm to reach the egg. Not only is the female gametophyte completely sheltered (never leaving the megasporangium or the confines of the ovule) and cared for by its parent sporophyte, but also the next sporophyte generation develops to the point of being an embryo within the now-mature seed and is potentially able to remain in an inactive state until environmental conditions are favorable for germination and establishment. - Then, the young sporophyte can continue utilizing the tissues of its female gametophyte parent as nutrition while it establishes a root and shoot system, giving the young conifer a head-start to establish its roots and shoot before it has to produce all of its food from photosynthesis.

Alternation of generations

All land plants, including bryophytes, have an alternation of generation. From here forward, we will see only this type of life cycle (each including all of the elements shown here). Be sure to study this cycle and understand it before interpreting the specific examples we will see in mosses, ferns, conifers, and flowering plants. There are two types of gametangia: antheridia (sperm-producing) and archegonia (egg- producing). In some vascular plants, including all seed plants, there are two types of sporangia as well, as we will see soon. - refers to having a multicellular organism in the haploid phase of the life cycle and a multicellular organism in the diploid phase of the life cycle; that is, mitosis occurs in both phases. - Gametophyte literally means the gamete-producing plant. - Gametophytes are haploid and produce gametes by mitosis. - Gametes fuse to form a diploid zygote, which develops into a new sporophyte. - Sporophyte literally means the spore-producing plant. Sporophytes are diploid and produce spores by meiosis. Spores germinate and develop into gametophytes.

Green Alga Life cycle Ulva

is unusual in having morphologically similar gametophyte and sporophyte generations. - We are introducing it here because it is relatively simple (in having morphologically similar gametophytes and sporophytes) compared to the alternation of generations in land plants, in which the gametophytes and sporophytes are morphologically distinct (in all modern representatives). - The alternation of generations in Ulva represents a distinct origin of that type of life cycle compared to the alternation of generations in land plants; - that is, land plants evidently did not evolve from an ancestral green alga that had this type of life cycle but instead they evolved an alternation of generations separately, from an ancestral green algal life cycle in which the only multicellular phase was haploid (and the zygote was the only diploid stage).