Why is starting new plants from seed called sexual propagation?

Sexual propagation is the reproduction of plants by seeds. The genetic material of two parents is combined by pollination and fertilization to create offspring that are different from each parent. There are several advantages of sexual propagation: It may be quicker and more economical than asexual propagation. Seeds are typically produced from sexual reproduction within a species, because genetic recombination has occurred. A plant grown from seeds may have different characteristics from its parents. Some species produce seeds that require special conditions to germinate, such as cold treatment.

Explanation:

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom

in situ hybridization has been widely used for research applications, including clinical cytogenetics, gene mapping, tumor biology and studies of chromosome evolution.