Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
Pollen competition in water birch (2x) styles. (A) Sperm cell nuclei in water birch (2x) and paper birch (6x) pollen tubes at equivalent stages of development. (B) Undivided generative cell nucleus in paper birch (6x) pollen tube growing near water birch (2x) pollen tube containing sperm cell nuclei. (Bar = 10 μm.)
