Capillary Tower
A Capillary Tower is one of the most important and crucial Tyranid organisms. Although harmless and immobile, these structures are literally the only link between entire Hive Fleets and the biomass they'd starve to death without.
Overview[edit]
Capillary Towers are Tyranid plants that reach the upper atmosphere (and beyond). Where they grow, Reclamation Pools then start to appear. The towers are linked to them via underground roots. Later, the towers link up with the sucking proboscis feeding tubes of Hive Ships in low orbit, which then pump the biomass upwards and distribute it to the waiting biovessels.
The capillary towers are capable of growing in the deepest of seas, able to sustain the pressure of the depths and the coldest of waters. As they grow the planet's tidal flow is altered by their powerful metabolic filtration systems, used to draw all biomass.
Tl;dr, giant umbilical cords used to feed entire bioships and keep the Nids from starvin' out. Although in Dawn of War II, they act as structures that buff the Tyranid's regenerative properties and overall durability. They are still made out of tissue paper however and can be easily destroyed.
Unfortunately, there are no rules for these things other than decoration.