Krynoid

Krynoids are a sapient form of vegetation which spreads itself between star systems and is considered a major pest throughout the galaxy. Their seeds travel in space within rock-like pods which can survive frozen for over 25 thousand years.

After establishing itself, the Krynoid never ceases growing until it covers the entire biosphere of the planet, consuming all animal life and merging itself with all previously existing vegetable life in the process. The Krynoid is a sapient lifeform, capable of communicating telepathically with other living things, both animal and vegetable, and forming a hive mind with the plants that surround it.

Biology
Krynoids launch their pods into space by means of violent explosions, and the pods always travel in pairs. After landing on a planet, they can remain frozen for over 25 thousand years and can be induced to hatch by the presence of UV radiation.

After the pod is open, the approach of a mobile life form will trigger the newborn Krynoid to launch itself onto the prey to infect it, consuming its body from inside and eventually turning the victim into a walking mass of vegetable matter: a new Krynoid is born. The transformation begins with the victim's skin turning into a moss-like covering and the presence of schizophytes (plant bacteria) in their bloodstream. The young Krynoid still hunts for food; only later in life will it become able to nourish itself via photosynthesis alone.

The adult Krynoid looks like an amorphous mass of mobile vegetable growth with huge tentacles. They are highly tolerant of frigid temperatures, but vulnerable to extreme heat. Thanks to quick consumption of animal matter and assimilation of vegetable matter, they can grow at an almost unbelievable rate. They are able to consume nitrogen directly from the atmosphere, an ability which on Earth is displayed only by a few specialized bacteria. They can also exert mental control over other species, sapient or otherwise.

Appearances

 * Doctor Who season 13 (Fourth Doctor) – The Seeds of Doom (1976)
 * BBV audio stories – The Root of All Evil (1999)
 * BBV audio stories – The Green Man (2002)
 * The New Eighth Doctor Adventures – Hothouse (2009)
 * Tales of Trenzalore - An Apple a Day (2014)