pollen grains are produced in male cones in a pine trees.
pollen grains are produced in male cones in a pine trees.
where are pollen grains produced in a pine trees?
male cones
conifers are heterosporous, generating two different types of spores: male microspores and female megaspores. 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 are 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 will 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 will give rise to the embryo, and a seed is produced. the female cone then opens, releasing the seeds which grow to a young seedling.
pinophyta
androecium are often faced by the pressures of sexual selection. this is particularly evident when pollen production, and there may be several sources of sexual selection on the ways that pollen is presented to pollen vectors. because pollen is packaged in units that ensure that several to many pollen grains travel together as in pollinia, polyads, viscin threads, etc., pollen donors may be able to monopolize stigmas and the associated ovules by blocking access by other males, unless selection also favors compensating stigma enlargement. furthermore, when pollen germination depends on some minimum number of pollen grains to overcome stigmatic inhibition ,which is a mechanism that heightens male competition, there may be selection for large pollen dispersal units or certain other pollen-dispensing mechanisms. if pollinator visits are few,bthen there is selection to package pollen in a way that all of it can be removed by one visit . when repeated pollinator visits are typical, there may be selection for various temporal patterns of pollen presentation and various methods of pollen dispensing. variation in stamen length, both within and among flowers influences pollen dispersal and potential male reproductive success. increasing the opportunity for paternity by distributing pollen among pollinators may take different routes in different systems, and any of these possibilities can be viewed in a sexual selection context . the gynoecium is also affected by sexual selection.  every part from the ovaries, styles, stigma, and carpels can be faced with the pressures of sexual selection. in ovule packaging, the intensity of pollen competition depends in part on the number of pollen grains relative to the number of ovules. although many factors may contribute to determining ovule number, one way for females to increase the level of pollen competition is to decrease ovule number while maintaining stigma size. the evolution of functional syncarpy (assuming no other attendant changes) presumably is a simultaneous increase in the number of ovules accessible from one stigma, which tended to decrease the intensity of pollen competition, and a concentration of pollen deposition on a single stigma, which tends to increase pollen competition. further increases in pollen competition could be brought about by an increase in pollen delivery (through change of pollinator, increased attractiveness of floral display, or rewards), a decrease in ovule number or stigma size, changes in temporal patterns of stigma receptivity, or changes in the competitive environment of the carpel .
sexual selection in flowering plants