HardBiologyHigh School (11–12)Diversity of Living Organisms › Gymnosperms and AngiospermsNumericalConceptual
A cycad has 2n = 22, and a coconut palm has 2n = 32.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A cycad has , and a coconut palm has . How many chromosomes are there in a cell of the food tissue of each seed? Explain why the two numbers are not found by the same rule.
Need a reminder? The key ideaShow
In angiosperms, ovules lie inside an ovary. The embryo sac has 7 cells and 8 nuclei: the egg, 2 synergids, 3 antipodals and the central cell with 2 polar nuclei. One pollen tube brings two male gametes. Syngamy (gamete and egg) gives the zygote, . Triple fusion (gamete and two polar nuclei) gives the primary endosperm nucleus, .
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The cycad. Its food tissue is the female gametophyte, which grew from a haploid megaspore: .
Step 2. The coconut. Its food tissue is the endosperm, formed by triple fusion of three haploid nuclei: .
Step 3. Why the rules differ. Only angiosperms have triple fusion. A gymnosperm seed stores food in its haploid gametophyte, made before fertilisation.
Answer: in the cycad and in the coconut; the coconut's endosperm is triploid, while the cycad's food store is the haploid female gametophyte.
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