Editing Conifer (section)

===Life cycle===

Conifers are [[heterosporous]], generating two different types of spores: male [[microspore]]s and female [[megaspore]]s. These spores develop on separate male and female [[sporophylls]] on separate male and female cones. In the male cones, microspores are produced from microsporocytes by [[meiosis]]. The microspores develop into pollen grains, which contain the male gametophytes. Large amounts of pollen are released and carried by the wind. Some pollen grains will land on a female cone for pollination. The generative cell in the pollen grain divides into two [[haploid]] sperm cells by [[mitosis]] leading to the development of the pollen tube. At fertilization, one of the sperm cells unites its haploid nucleus with the haploid nucleus of an egg cell. The female cone develops two ovules, each of which contains haploid megaspores. A megasporocyte is divided by meiosis in each ovule. Each winged pollen grain is a four celled male [[gametophyte]]. Three of the four cells break down leaving only a single surviving cell which develop into a female [[multicellular]] gametophyte. The female gametophytes grow to produce two or more [[archegonia]], each of which contains an egg. Upon fertilization, the [[diploid]] egg gives rise to the embryo, and a seed is produced. The female cone then opens, releasing the seeds which grow to a young [[seedling]].

# To fertilize the ovum, the male cone releases [[pollen]] that is carried in the wind to the female cone. This is [[pollination]]. (Male and female cones usually occur on the same plant.)
# The pollen fertilizes the female gamete (located in the female cone). Fertilization in some species does not occur until 15 months after pollination.<ref>{{Cite web |url=http://bioserv.fiu.edu/~biolab/labs/1011/Spring%202009/TA%20notes%20and%20pictures/Week%205%20-%20Seed%20Plants.htm |title=Gymnosperms |access-date=2014-05-11 |archive-date=2015-05-27 |archive-url=https://web.archive.org/web/20150527001621/http://bioserv.fiu.edu/~biolab/labs/1011/Spring%202009/TA%20notes%20and%20pictures/Week%205%20-%20Seed%20Plants.htm |url-status=dead }}</ref>
# A fertilized female gamete (called a [[zygote]]) develops into an [[embryo]].
# A [[seed]] develops which contains the embryo. The seed also contains the integument cells surrounding the embryo. This is an evolutionary characteristic of the [[Spermatophyta]].
# Mature seed drops out of cone onto the ground.
# Seed germinates and seedling grows into a mature plant.
# When the plant is mature, it produces cones and the cycle continues.

==== Female reproductive cycles ====
Conifer reproduction is synchronous with seasonal changes in temperate zones. Reproductive development slows to a halt during each winter season and then resumes each spring. The male [[strobilus]] development is completed in a single year. Conifers are classified by three reproductive cycles that refer to the completion of female strobilus development from initiation to seed maturation. All three types of reproductive cycle have a long gap between [[pollination]] and [[fertilization]].

'''One year reproductive cycle''': The genera include ''[[Abies]]'', ''[[Picea]]'', ''[[Cedrus]]'', ''[[Pseudotsuga]],'' ''[[Tsuga]]'', ''[[Keteleeria]]'' ''([[Pinaceae]])'' and ''[[Cupressus]], [[Thuja]], [[Cryptomeria]], [[Cunninghamia]]'' and ''[[Sequoia (genus)|Sequoia]] ([[Cupressaceae]])''. Female strobili are initiated in late summer or fall of a year, then they overwinter. Female strobili emerge followed by pollination in the following spring. Fertilization takes place in summer of the following year, only 3–4 months after pollination. Cones mature and seeds are then shed by the end of that same year. Pollination and fertilization occur in a single growing season.<ref name="Singh-1978">{{cite book |last=Singh |first=Hardev |title=Embryology of Gymnosperms |date=1978 |publisher=Gerbrüder Borntraeger |isbn=978-3-443-14011-3 }}{{pn|date=January 2025}}</ref>

'''Two-year reproductive cycle''': The genera includes ''[[Widdringtonia]]'', ''[[Sequoiadendron]]'' (''[[Cupressaceae]]'') and most species of ''Pinus''. Female [[strobilus]] initials are formed in late summer or fall then overwinter. Female strobili emerge and receive pollen in the first year spring and become conelets. The conelet goes through another winter rest and, in the spring of the second year [[archegonia]] form in the conelet. Fertilization of the archegonia occurs by early summer of the second year, so the pollination-fertilization interval exceeds a year. After fertilization, the conelet is considered an immature cone. Maturation occurs by autumn of the second year, at which time seeds are shed. In summary, the one-year and the two-year cycles differ mainly in the duration of the pollination-fertilization interval.<ref name="Singh-1978"/>

'''Three-year reproductive cycle''': Three of the conifer species are [[pine]] species (''[[Pinus pinea]]'', ''[[Pinus leiophylla]]'', ''[[Pinus torreyana]]'') which have pollination and fertilization events separated by a two-year interval. Female strobili initiated during late summer or autumn of a year, then overwinter until the following spring. Female [[strobili]] emerge then pollination occurs in spring of the second year then the pollinated strobili become conelets in the same year (i.e. the second year). The female [[gametophytes]] in the conelet develop so slowly that the [[megaspore]] does not go through free-nuclear divisions until autumn of the third year. The conelet then overwinters again in the free-nuclear female gametophyte stage. Fertilization takes place by early summer of the fourth year and seeds mature in the cones by autumn of the fourth year.<ref name="Singh-1978"/>

==== Tree development ====
The growth and form of a forest tree are the result of activity in the primary and secondary [[meristem]]s, influenced by the distribution of photosynthate from its needles and the hormonal gradients controlled by the apical meristems.<ref name="Fraser-1964">{{cite journal|last1=Fraser|first1=D.A.|last2=Belanger|first2=L.|last3=McGuire|first3=D.|last4=Zdrazil|first4=Z.|year=1964|title=Total growth of the aerial parts of a white spruce tree at Chalk River, Ontario, Canada|journal=Can. J. Bot.|volume=42|issue=2 |pages=159–179|doi=10.1139/b64-017 |bibcode=1964CaJB...42..159F }}</ref> External factors also influence growth and form.

Fraser recorded the development of a single white spruce tree from 1926 to 1961. Apical growth of the stem was slow from 1926 through 1936 when the tree was competing with [[herb]]s and [[shrub]]s and probably shaded by larger trees. Lateral branches began to show reduced growth and some were no longer in evidence on the 36-year-old tree. Apical growth totaling about 340 m, 370 m, 420 m, 450 m, 500 m, 600 m, and 600 m was made by the tree in the years 1955 through 1961, respectively. The total number of needles of all ages present on the 36-year-old tree in 1961 was 5.25&nbsp;million weighing 14.25&nbsp;kg. In 1961, needles as old as 13 years remained on the tree. The ash weight of needles increased progressively with age from about 4% in first-year needles in 1961 to about 8% in needles 10 years old. In discussing the data obtained from the one 11 m tall white spruce, Fraser et al. (1964)<ref name="Fraser-1964" /> speculated that if the photosynthate used in making apical growth in 1961 was manufactured the previous year, then the 4&nbsp;million needles that were produced up to 1960 manufactured food for about 600,000&nbsp;mm of apical growth or 730 g dry weight, over 12&nbsp;million mm<sup>3</sup> of wood for the 1961 annual ring, plus 1&nbsp;million new needles, in addition to new tissue in branches, bark, and roots in 1960. Added to this would be the photosynthate to produce energy to sustain respiration over this period, an amount estimated to be about 10% of the total annual photosynthate production of a young healthy tree. On this basis, one needle produced food for about 0.19&nbsp;mg dry weight of apical growth, 3&nbsp;mm<sup>3</sup> wood, one-quarter of a new needle, plus an unknown amount of branch wood, bark and roots.

The order of priority of photosynthate distribution is probably: first to apical growth and new needle formation, then to buds for the next year's growth, with the cambium in the older parts of the branches receiving sustenance last. In the white spruce studied by Fraser et al. (1964),<ref name="Fraser-1964" /> the needles constituted 17.5% of the over-day weight. Undoubtedly, the proportions change with time.