Sex is determined by the pollen as it germinates the seed.
read this:
plant sex is determined by sperm, just like in animals. most are fert by two sperm, one sperm fert the egg and the other sperm ferts the endosperm and polar nuclei. (the endosperm is what the seedling eats for nutrients before it can photosynthesize). the sperm that ferts the egg will determine the sex of the plant.
of course you may be thinking "well then why is it that if you stress a plant youll get a hermie or whatever" wellllll in response to the stress hormone regulation is thrown off balance and allows for male phenotypic expression to show up. but you have to note, before that hormone stress, that plant was a female.

as far as isolating hormones... next to impossible. identifying hormones in a mess of other organic materials, possible, been done, many times, just cant be physically separated from the rest of the slew and be separated purely at that..

the pulp you are referring to is largely parenchyma and ground tissue. that pulp is what is referred to as totipotent and undifferentiated. basically, it can turn into anything. given that, i really dont see that happening, you might be able to turn a male into a hermie... maybe. the determining factor for the sex of the plant is the chromosomal configuration that comes from mitotic division in the pollen grain. pollen is a living plant. a living haploid plant. thats kind of hard to understand unless youre versed in plants life cycles. it an alternation of generation cycle, completely different from animals. as if you sperm could grow up and produce babies... kind of. lol.

im not going to go over the whole thing, but this is a quick run down. the actual plant that you think of, a tree for example, is considered a sporophyte. the sporophyte produces spores through meiosis. this means that there is a reduction devision in choromosomes and therefore ploidy number. the spore are what we call pollen that come from male sporangia(spore producing region of a plant) ok, here is a recap, sporophyte has a diploid chromosome number, sporangia produce spores through reduction division meiosis, which means that the spore(pollen) is haploid,and has genetic recombination for a different gene set with every one.

the pollen produces two sperm, like a said before, but unlike animals, the pollen produces these two sperm through mitotic divisions, which means the two sperm each retain their haploid chromosome nature and dont go through another reduction division.
basically this all comes down to the sperm from the pollen determine sex of the plant.
more from the biology thread by asf2j

"Femenized" seeds come from a female plant being pollinated by pollen from a hermie right?
Undoubtably the hormonal shift that takes place due to stress that causes the hermie also produces a larger number of female plant sperm (pollen)
or that would seem to be the case.

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