Plant 43

Plant 43 (unofficially named as "Ivy") is the result of Umbrella's experiments to create a more sophisticated version of Plant 42, which it was based on.

Biology
"Ivies" are bipedal green plants with arm-like vines and a large orange flower for a 'head'. Sentient, or close to sentience, their main method of attack is to whip their target with their vines. They can also attack at range by spitting acid. If the prey gets too close, the plant is able to grab them with their vines and spit acid over their bodies. In some cases, the monster pretends to be dead, but would still whip its target if they walk too close by it.

The experimental anti-B.O.W. gas "P-Epsilon" cannot defeat P-43 and will instead cause it to mutate. This mutated form is physically tougher than its original counterpart, and its acid has become poisonous to humans.

During the outbreak
During the Raccoon City Outbreak, Ivies were encountered by Leon S. Kennedy and Claire Redfield as they explored the Raccoon City Underground Research Facility, and by HUNK during his last-minute escape from the Raccoon City Police Department. Umbrella presumably continued at least low-level production of Ivies, or researched into them for several years afterwards, as several were encountered by Chris Redfield, Jill Valentine and Albert Wesker during their respective infiltrations of the Umbrella Caucasus Research Facility in 2003.

Strategy
Ivies are very slow to move, and can be fought at range with a handgun in most of the games they appear in. In a close range fight however, they are weak against fire and will die quickly when attacked with a Flamethrower or Flame Rounds from a Grenade Launcher.

Ivies display a unique weak spot, gunfire to the center of the opened flower will kill it instantly. It's actually possible to kill an Ivy with two pistol shots. The first will cause it to rear back, and then open its flower-head for a moment, and a second into the weak spot will kill it immediately. This only works in Umbrella Chronicles due to it's aiming system

Ivy + YX
A tropical variant of Plant 43 was encountered by Leon S. Kennedy and Jack Krauser in 2002's South America.

Ivy + YX possessed a slightly more human lower body, and perhaps consequently, was somewhat faster moving. Its head shared a close resemblance to tropical flowers such as the venus flytrap, with three long narrow 'jaws' that flare out wider at the tip. The Ivy + YX was also stronger and tougher than its original counterpart, and lacked the aforementioned 'weak spot'.

Trivia

 * Both B.O.W.s were created by analyzing field data taken from Plant 42. This makes the "43" a "given name" rather than its parent's "...At sector 42".
 * Plant 43 may have been inspired by the man-eating plant from the movie, "The Little Shop of Horrors".