Editing Botany (section)

== Plant evolution ==
{{Main|Evolutionary history of plants}}
[[File:Rhynia stem.jpg|thumb|left|alt=colour image of a cross section of a fossil stem of ''Rhynia gwynne-vaughanii'', a Devonian vascular plant|Transverse section of a fossil stem of the Devonian vascular plant ''[[Rhynia]] gwynne-vaughani'']]
The [[chloroplast]]s of plants have a number of biochemical, structural and genetic similarities to [[cyanobacteria]], (commonly but incorrectly known as "blue-green algae") and are thought to be derived from an ancient [[endosymbiotic theory|endosymbiotic]] relationship between an ancestral [[eukaryote|eukaryotic cell]] and a [[Chloroplast#Parent group: Cyanobacteria|cyanobacterial resident]].{{sfn|Mauseth|2003|pp = 552–581}}{{sfn|Copeland|1938|pp = 383–420}}{{sfn|Woese et al.|1977|pp = 305–311}}{{sfn|Cavalier-Smith|2004|pp = 1251–1262}}

The [[algae]] are a [[Polyphyly|polyphyletic]] group and are placed in various divisions, some more closely related to plants than others. There are many differences between them in features such as cell wall composition, biochemistry, pigmentation, chloroplast structure and nutrient reserves. The algal division [[Charophyta]], sister to the green algal division [[Chlorophyta]], is considered to contain the ancestor of true plants.{{sfn|Mauseth|2003|pp = 617–654}} The Charophyte class [[Charophyceae]] and the land plant sub-kingdom [[Embryophyta]] together form the [[monophyletic]] group or clade [[Streptophytina]].{{sfn|Becker|Marin|2009|pp = 999–1004}}

Nonvascular land plants are [[embryophyte]]s that lack the vascular tissues [[xylem]] and [[phloem]]. They include [[moss]]es, [[Marchantiophyta|liverworts]] and [[hornwort]]s. [[Pteridophyte|Pteridophytic]] vascular plants with true xylem and phloem that reproduced by spores germinating into free-living gametophytes evolved during the Silurian period and diversified into several lineages during the late [[Silurian]] and early [[Devonian]]. Representatives of the lycopods have survived to the present day. By the end of the Devonian period, several groups, including the [[Lycopodiophyta|lycopods]], [[Sphenophyllales|sphenophylls]] and [[progymnosperm]]s, had independently evolved "megaspory" – their spores were of two distinct sizes, larger [[megaspore]]s and smaller microspores. Their reduced gametophytes developed from megaspores retained within the [[sporangium|spore-producing organs]] (megasporangia) of the sporophyte, a condition known as endospory. Seeds consist of an endosporic megasporangium surrounded by one or two sheathing layers ([[integument]]s). The young sporophyte develops within the seed, which on [[germination]] splits to release it. The earliest known seed plants date from the latest Devonian [[Famennian]] stage.{{sfn|Fairon-Demaret|1996|pp = 217–233}}{{sfn|Stewart|Rothwell|1993|pp = 279–294}} Following the evolution of the seed habit, [[Spermatophyte|seed plants]] diversified, giving rise to a number of now-extinct groups, including [[Pteridospermatophyta|seed ferns]], as well as the modern gymnosperms and angiosperms.{{sfn|Taylor|Taylor|Krings|2009|loc = chapter 13}} [[Gymnosperm]]s produce "naked seeds" not fully enclosed in an ovary; modern representatives include [[Pinophyta|conifers]], [[cycad]]s, ''[[Ginkgo]]'', and [[Gnetophyta|Gnetales]]. [[Angiosperms]] produce seeds enclosed in a structure such as a [[Gynoecium|carpel]] or an [[ovary]].{{sfn|Mauseth|2003|pp = 720–750}}{{sfn|Mauseth|2003|pp = 751–785}} Ongoing research on the molecular phylogenetics of living plants appears to show that the angiosperms are a [[sister clade]] to the gymnosperms.{{sfn|Lee|Cibrian-Jaramillo|Kolokotronis|Katari|2011|p = e1002411}}