Chapter 30 Plant Diversity II The Evolution of Seed Plants Overview Feeding the World The seed arose about 360 million years ago Seed plants including gymnosperms and angiosperms have come to dominate modern landscapes and make up the great majority of plant biodiversity Agriculture the cultivation and harvest of plants especially angiosperms began 13 000 years ago Humans began the cultivation of plants independently in various regions including the Near East East Asia Africa and the Americas This was the single most important cultural change in the history of humanity and it made possible the transition from hunter gatherer societies to permanent settlements Concept 30 1 The reduced gametophytes of seed plants are protected in ovules and pollen grains A number of terrestrial adaptations contributed to the success of seed plants These adaptations include the seed the reduction of the gametophyte generation heterospory ovules and pollen Bryophyte life cycles are dominated by the gametophyte generation while seedless vascular plants have sporophyte dominated life cycles The trend to gametophyte reduction continued in the lineage of vascular plants that led to seed plants Seedless vascular plants have tiny gametophytes that are visible to the naked eye The gametophytes of seed plants are microscopically small and develop from spores retained within the moist sporangia of the parental sporophyte In seed plants the delicate female gametophyte and the young sporophyte embryo are protected from many environmental stresses including drought and UV radiation The gametophytes of seed plants obtain nutrients from their parents while the free living gametophytes of seedless vascular plants must fend for themselves Heterospory is the rule among seed plants Nearly all seedless plants are homosporous producing a single kind of spore that forms a hermaphroditic gametophyte Seed plants likely had homosporous ancestors All seed plants are heterosporous producing two different types of sporangia that produce two types of spores Megasporangia produce megaspores which give rise to female egg containing gametophytes Microsporangia produce microspores which give rise to male sperm containing gametophytes Seed plants produce ovules In contrast to the few species of heterosporous seedless vascular plants seed plants are unique in retaining their megaspores within the parent sporophyte Layers of sporophyte tissue integuments envelop and protect the megasporangium Gymnosperm megaspores are surrounded by one integument Angiosperm megaspores are surrounded by two integuments An ovule consists of the megasporangium megaspores and integuments A female gametophyte develops from a megaspore and produces one or more egg cells Pollen eliminated the liquid water requirement for fertilization Pollen grains are covered with a tough coat containing sporopollenin They are carried by wind or animals The transfer of pollen to the vicinity of the ovule is called pollination The microspores develop into pollen grains that are released from the microsporangium The pollen grain germinates and grows as a pollen tube into the ovule where it delivers one or two sperm into the female gametophyte Bryophytes and seedless vascular plants have flagellated sperm cells that swim a few centimeters through a film of water to reach the egg cells within the archegonium In seed plants the female gametophyte is retained within the sporophyte ovule Male gametophytes travel long distances as pollen grains The sperm of seed plants lack flagella and do not require a film of water as they rely on the pollen tube to reach the egg cell of the female gametophyte within the ovule The sperm of some gymnosperm species retain the ancestral flagellated condition providing evidence of this evolutionary transition The evolution of pollen contributed to the success and diversity of seed plants Seeds became an important means of dispersing offspring What is a seed When a sperm fertilizes an egg of a seed plant the zygote forms and develops into a sporophyte embryo The ovule develops into a seed consisting of the embryo and its food supply within a protective coat derived from the integuments The evolution of the seed enabled plants to resist harsh environments and disperse offspring more widely For bryophytes and seedless vascular plants single celled spores are the only protective stage in the life cycle Moss spores can survive even if the local environment is too cold too hot or too dry for the moss plants themselves to survive Because of their tiny size the spores themselves can be dispersed in a dormant state to a new area Spores were the main way that plants spread over Earth for the first 100 million years of life on land The seed represents a different solution to resisting harsh environments and dispersing offspring In contrast to a single celled spore a multicellular seed is a much more complex resistant structure After being released from the parent plant a seed may remain dormant for days or years Under favorable conditions it germinates and the sporophyte embryo emerges as a seedling Concept 30 2 Gymnosperms bear naked seeds typically on cones The ovules and seeds of gymnosperms naked seeds develop on the surfaces of modified leaves that usually form cones strobili In contrast ovules and seeds of angiosperms develop in enclosed chambers called ovaries The most familiar gymnosperms are the conifers cone bearing trees such as pine fir and redwood The four phyla of extant gymnosperms are Cycadophyta Ginkgophyta Gnetophyta and Coniferophyta There are four plant phyla grouped as gymnosperms Phylum Ginkgophyta consists of only a single extant species Ginkgo biloba This popular ornamental species has fanlike leaves that turn gold before they fall off in the autumn Landscapers usually plant only male trees because the coats of seeds produced by female plants produce a repulsive odor as they decay Cycads phylum Cycadophyta have large cones and palmlike leaves 130 species of cycads survive today Cycads flourished in the Mesozoic era which was known as the Age of Cycads Phylum Gnetophyta consists of three very different genera Weltwitschia plants from deserts in southwestern Africa have straplike leaves that are among the largest known leaves Ephedra Mormon tea is a shrub of the American deserts Gentum species are tropical trees or vines The conifers belong to the largest gymnosperm phylum the phylum Coniferophyta The term conifer comes from the