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In over 60% of angiosperms, pollen grains are shed off at 2-celled stage (Vegetative cell and a generative cell) and in the other 40% it is shed off at 3-celled stage (Vegetative cell and 2 male gametes). During the pollen pistil interaction, if the pollen grain is 2- celled, it divides to form a 3-celled stage during the growth of pollen tube in the stigma, while as the 3-celled directly goes into the ovary/ovule. I don't understand the need for the pollen grains to remain in the 2-celled stage if, ultimately, they convert into a 3-celled stage. Why don't the pollen grains remain in the 3-celled stage from the very beginning? What is the advantage of being in the 2-celled stage before pollination?

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  • $\begingroup$ This is an interesting question and I've upvoted it. It would be better if you could provide a source for your claim, so that you don't send someone (like me) on a wild goose chase. Thanks! :) (I like to find the answers to interesting questions, as it's fun to learn new things, but I'd like to know I'm spending time on something that's a fact.) $\endgroup$ Jul 13, 2023 at 13:43
  • $\begingroup$ @anongoodnurse Thank you for the upvote. As for the source of my claim, I read that in the textbook issued to me by my school, and you can find that here. It is on page 7 and 17 (to save you some time :>). $\endgroup$ Jul 13, 2023 at 15:06
  • $\begingroup$ Thanks for providing a link! As you're new here, you might not have understood that the source, a link and a quote should be edited into the question. That insures that the question is self-contained. Again, though, thanks! That chapter discusses things I never knew. $\endgroup$ Jul 14, 2023 at 0:35
  • $\begingroup$ Thanks for letting me know. And, you're welcome. :) $\endgroup$ Jul 14, 2023 at 5:12

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Since, the topic itself is under research i would refrain from making bold assumptions but my hypothesis (based on evolutionary benefit; extra nutrient availability in pistil) is plausible enough and helpful for understanding your topic, but before that let me explain to you some prerequisites since you would need that understanding to think beyond as well. (Your answer is in last 2 paragraphs.)

  1. PPI: Pollen Pistil Interaction:

As the pollen lands on the stigma (which is relatively moist) pollen absorbs some of moisture and it's metabolic activity is triggered resulting in pollen tube growth (obviously there are intermediate steps like reorientation of pollen, which in itself involves many extra cellular messengers (ECM) generaly sucrose gradient or some other peptide gradients and they trigger intra cellular signals which is the area of focus in your answer).

  1. PT : Pollen Tube:

Consider it like a Big Long Tube (not hollow but contains actin filaments/shank actin filaments and its mechanism of extension in pollen tube is like reverse-fountain cytoplasmic streaming don't be afraid of the term it's simply like that - at axis/center of tubule its convoluted into itself while extending forward at the peripheral portion thus pushing ahead into the pistil. Pollen tube isn't like shoot hair which is often misunderstood as similar (shoot hair is continuous projection of single cell with no blocking in between, but in PT there is callous deposit to prevent back flow) for images and further explanation refer this https://www.frontiersin.org/articles/10.3389/fpls.2020.589712/full

  1. Progamic Phase: It is the phase between pollen landing on pistil and fertilisation. That is Period of Pollen Pistil Interaction & Pollen Tube Elongation.

  2. Does cell while moving into pollen tube derives any nutrient from pistil? In my opinion it is very unlikely and there are rarely any conclusive research papers out there to confirm or discard that entirely BUT it might be dependent on some signaling molecules for his maneuvering and metabolic activities and it is strongly established. Now, coming to your question

Your question is like that:

Why pollen's spermatic/generative cell divides in 'progamic phase' instead of already divided beforehand during the 'gametogenesis' phase itself.

Answer could be sought in Evolutionary benefits, why to spend so much of energy and resource in dividing all pollen cells into 3 celled stage when most of them are ultimately not going to succeed thus leaving this for the progamic phase to do so.

Also, to keep pollen 'effectively' light weighted this is beneficial, since loading pollen with vegetative cell will increase its survival time under outside environment we can not compromise on that part, thus if sperm cell is divided into the parent body itself it will lead to increase in overall weight. (Read last paragraph again).

Availability of nutrient in pistil - such hypothesis is dependent on point number 4 in prerequisites list. I could not conclude it immediately but it could be turning point to the answer.

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