In Fungi
There are a variety of saprophytic and parasitic organisms that have evolved the habit of growing into their substrates as thin strands. This is most typical of the "true" fungi, but it has also evolved in actinobacteria (bacteria), oomycetes (stramenopiles, like kelp), parasitic plants, and rhizocephalans (parasitic barnacles).
Read more about this topic: List Of Examples Of Convergent Evolution